Figure your route and buy a small GPS receiver unit, like a Garmin Etrex Legend. Go to www.terracaching.com and www.geocaching.com and find caches hidden along the way. Load up at the Dollar Store on small trinkets (swag) to swap for in the caches.
Caching has taken me to some of the most interesting places that I never would have gone to otherwise.
Having driven Route 80 out to Utah, I have to agree that there are big stretched of DULL along the way. Most of Nebraska after you leave the Platte River gets pretty flat and uninteresting. Cheyenne and Rock Springs and Little America in Wyoming are interesting, but there’s a lot of nothing in between. Going west of Salt Lake is kinda dull for a long time, too (although it’s worth seeing the salt flats and the Big Tree), continuing to be dull across northern Nevada. Frequent side trips may halp, but you have to weigh that against the extra time needed.
That said, the stuff along the route is well worth seeing. Just think of the early settlers, who didn’t have paved roads and automobiles.
How about doing the Mother Road. We did Rt 66 from St. Louis to Ca. last summer. It was awesome, all the kitchy stuff and goofy landmarks; totem poles, dinosaurs, teepees, carhenge, etc. Don’t forget Sante Fe, I especially recommend the Thousand Waves Spa. We did as much of old 66 as we could with even older 66 off to the right the whole way. Most of the time we didn’t go faster than 55 mph. Driving the old highway with no yellow line in the middle and curbs on either side was a real trip. We were the only ones on the old road for hundreds of miles and watched the guys in a hurry zooming past on the “new” road only a couple of dozen feet away.
On the way home, via Yosemite, Grand Teton, Glacier, we ate breakfast at Wall Drug and went to the “Corn Palace”: Americana at it’s finest. 
We’re doing the Grand Circle this summer and probably back to Grand Teton (my favorite NP).
Don’t let people put you off to the National Parks, buy a pass and go. Yeah, they can be crowded, but good God they are awesome!!!
Yeah, but of you make it all the way through you can see:
The Worlds Largest Prairie Dog
I also seem to remember a sign advertising:
“Two headed calf (Live!).”
So, don’t be thinking there ain’t sights to see! 
Different folk have different ideas about what is exciting. What do you like? If you live in a large metropolitan area, then perhaps some of those “boring” areas in the heartland might very well be quite interesting to you. Personally, I have had plenty of views that stretch from horizon to horizon over rolling hills but I have met some folks who were blown away by it.
Things of interest in Iowa and Nebraska along I-80 (most have been personally tested for family fun. Stay near the interstate in Iowa. A great place to live, but many people find eating at small town diners where you are the only one from out of town to be… unnerving. If you want to see small town living in Iowa at it’s best, participate in RAGBRAI, the annual bike ride across Iowa.) Driving time between sites listed here is about 1-3 hours each.
Iowa City with the U of Iowa- Best place in Iowa for a wide array of ethnic food. First capitol of Iowa with historical display. (Plan about 30 minutes for this before dinner.) Pick up exotic selection of beer from the widest choice of Belgian ales west of the Mississippi from John’s Grocery. Do not drink them on the sidewalk in front of the store, no matter how tempting they look.
Detour up 380 to Cedar Rapids and go to the Czech museum.
Amana colonies west of Iowa City - good homestyle German food served family style. Bring a large appetite. Tour the various old timey factories and pick up a bottle of dandelion wine. Come on, you know you want to see what it tastes like. Don’t stay in the furniture factory too long or your wife will pick out a high quality custom handmade dining room table set to be delivered to your house (not to be confused with the now mass marketted “Amish” furniture you find elsewhere. The Amana colonies have many Mennonites and Amish living nearby but were actually a German commune up until like 1920.)
Des Moines - Spend the day at the Living History Farms. Really, it’ll take most of teh day. Experience a frontier farm, turn of the century farm, and 1870’s frontier village. Get there on the right day and you can help scrape hides, plow a field behind a horse, or see an 1890’s baseball game. Learn about the evolution of farming up to the present day, more interesting than it sounds!
Omaha NE- Henry Doorly Zoo. Where else can you see penguins, a rainforest and a desert in the midwest?
Kearney NE- at the right time of year witness the migration of sandhill cranes. See the sky fill with cranes and eagles and learn about their migration in the gift shop.
Grand Island NE- Stuhr museum of prairie pioneers (more museumy than Living history farm, but very interesting.)
If you are driving at night at this point, pick a random exit and go 10 miles off the interstate. Stop and get out of the car. See more stars than many of you have ever seen before. Listen to the silence. Hear a coyote howl, note that you have no cell phone coverage, get a little nervous wondering if that was a coyote or a wolf, and head back for the interstate. Note that the interstate exit will not have a gas station. Pay attention to the signs saying “next gas station 100 miles.”
North Platte NE - home of Buffalo Bill! I haven’t stopped there, but they have to have a museum about it. I mean, it’s Buffalo Bill!
I haven’t found anything in eastern Colorado yet. Obviously Denver has all the big city stuff. Once you get west of denver, get off the interstate. Lots of scenic winding drives, isolated river banks to picnic by, etc.
If you want to go from Vegas to the coast the long way, you can go through Death Valley. It’s pretty stunning. The sweep of the valley up to the mountaintops is just incredible. I imagine it’s hot as hell in the middle of summer, but then you can say you experienced that and lived to tell the tale. From the west side of Death Valley you go south through the Owens Valley under the Sierras, then head west along the Kern river. It’s like a paradise after so much time in the desert.
In general, once you get into the mountains, say, past the front range in Colorado I don’t see much of a point in staying on the interstates unless you really have to be somewhere or it’s the only way to go.
When I was a lad I took three weeks to drive from NJ to the west coast to deliver a neighbor’s pickup truck to be shipped to Hawaii. I spent a few days in Boulder, CO, then went out to Moab, then down to the Grand Canyon, over to Vegas, out to California like I said above, then up to the bay area and saw some of San Francisco and some parks north of there, then to Oakland, where I dropped off the truck and flew back to Denver.
The Sandhill Cranes are amazing, but they go through Kearney in March.
I’m FROM Nebraska, so I know about flat and boring. Even flying home means 2-3 hours of driving for me. Even if I feel a little insulted by all the slagging on the dullness of driving the the nation’s breadbasket, I agree that it can be monotonous.
I think a few things would really help. One is getting off the interstate, at least for parts of it. Get a good guidebook and actually do some of the stuff in it. Drive into the towns and eat at some diners and local places. Learn more about the history of the area. Some of I-80 follows the Oregon trail, for example. Some states have audio tours you can check out from their rest stops and leave off before you cross the border, telling you a lot of this stuff. You can do a lot to turn it from “That hellacious endless flatland we couldn’t drive through fast enough” to something worth remembering.
Some other thoughts: Check out where and when the minor league teams are playing. Fun way to kill an evening (not just applicable to the midwest). Ditto the various ethnic festivals and fairs that happen in small towns across the midwest. Des Moines has a pretty interesting art museum, worth a stop. Omaha’s zoo is really good, one of the best “family zoos” in the country. Also fairly handy off I-80 in Nebraska are the Strategic Air Command museum, Stuhr (already mentioned) and the Museum of Nebraska Art. And there is Pioneer Village, god help us all.
Oh, and this is expensive, but consider the train. Very relaxing way to travel; get a sleeper car. It isn’t cheap, but your meals are included and it’s a really neat way to see the country. FWIW, the trains from Chicago usually have you conveniently sleeping through the more “boring” parts of the plains states.
Given they’re flying back, yes depending on how long you stop etc. 20+ years ago, Typo Knig and I took a 4 week cross-country trip - from North Carolina up to Seattle, then down the coast and back along the southerly route.
We visited:
St. Louis (just long enough to see the Gateway Arch - cool sight as you’re driving west through southern IL). Colorado Springs/Pikes Peak/Garden of the Gods (1 day), Rocky Mountain National Park (1 day), Grand Teton / Yellowstone (4-5 ish days), then a 2 day drive to Seattle and a few days around there, then a leisurely drive along the coast to the SF Bay area. Saw the giant redwoods en route to SF - very impressive. From SF, 4 and a half long driving days home. Total time, 27 days. Admittedly, it was just two of us, both adults - kids would make the long-driving days less productive because you’d have to stop more often.
Part of our trip was visiting people in the various locations - if you’re purely sightseeing you might spend less time in places like Seattle, and more doing the national parks etc.
We would recommend visiting any of those. Places we wish we’d visited since we were “near”: Crater Lake (Oregon? memory is fuzzy), Grand Canyon (we drove back along I-40 and I didn’t realize how close we were, but we were pointed homeward at that point so we didn’t opt to take a day side trip).
Other places worth the visit: Glacier National Park (northern Montana) - we visited that the next year. Mount Rushmore sounds cool (I’ve never been).
I second this! Glenwood Canyon Highway is the most beautiful Interstate highway, hands down. Make sure your car’s good at high altitude, though – Eisenhower Tunnel is at 11,000+ feet!
Indeed you could spend all three weeks driving around the West, and never run out of sights to see or things to do.
Sadly, U.S. 666 no longer exists. 
Everyone seems to be suggesting I-70 over I-80, and I can’t disagree, (and I live on I-80), but may I make another suggestion on the Wyoming issue. Take I-90 through the Black Hills, and plan to spend tons of time there. The Black Hills are wonderful. Then travel to Sheridan, Wyoming; travel over some gorgeous mountains (or slip north and visit the sight formerly known as Custer’s Battlefield in MT., or South and stay at the hot pools in THermopolis, WY) and end up in Cody, Wyoming. A great town to visit. Then through Yellowstone (don’t rust through this either); then through Jackson Hole by the Tetons (stop in Wilson, ID and see the Bar-J Wranglers, we travel 200 miles to see them every year), then into Utah and head South to Bryce Canyon and Zion National Park.
If you must take I-80, listen to **Kilvert’s Pagan ** and take the Snowy Range Road, stop in Saratoga, Wyoming and soak in the free outdoor hot pools. Then stop and see me when you’re in SW Wyoming.
Make sure the car you rent has satellight radio, or you’ll wish you did. Often.
Get off the interstate. If you take this advice, be sure to have a good radar detector.
Kansas has surprisingly good Mexican food. But avoid The Jayhawk Motel like an open wound on a Hatian crack whore.
Don’t fly back to New Jersey. 
Great Sand Dunes National Park was incredible and amazing. Definitely worth going out of your way to see.
