I happen to live fairly close to Gettysburg. I’ve given battlefield tours to more visiting friends and relatives than I can count. I usually drive them around the battlefield, then stop at the museum at the end. Sometimes we’ll have dinner in one of the historic buildings. A couple of times I’ve been lucky and just happened to catch reenactors who happened to be there for the weekend.
Milwaukee’s crown jewel is our Art Museum, but it doesn’t really encapsulate the city’s character. If I only had one place to go, I’d choose a tour of Lakefront Breweryfollowed by their famous Friday Fish Fry.
Cool topic, Reply! I live in Rhode Island. I will take people to Boston, Newport and the southern coast of Rhode Island and make sure they see the ocean.
I did the Freedom Trail in Boston with one buddy, which was a nice workout. M Reserve unit is in the North End of Boston, so since enlisting, I’ve found a lot of little gems there. Including Ernesto’s, where you can get a slice a pizza (which is really a quarter of the pie) and a soda for $5!
On the southern coast of RI, I’ll take them to Galilee, which obviously is across the water from Jerusalem!
And pretty much all of Newport is pretty cool. One buddy was particularly intrigued by what’s called the Viking Tower.
I live in the DC area, and usually folks already know the things they want to see as far as museums and monuments go, but I often take them to places like Rock Creek Park and Great Falls to show them the natural side of the area.
My favourite thing to do is take them out to Chinatown/Kensington Market/U of Toronto (they’re all within a very walkable distance from each other)
Itinerary might go a little like this:
Mid-morning, get up and head out for dim sum
Check out the shops in Chinatown (especially Tap Phong, the awesomest store ever)
Duck into Kensington Marketto poke around in some vintage shops (if it’s a Car-Free-Sunday, all the better)
Have some lunch/brunch at La Palette
Walk through the university campus and admire the architecture
End up on Harbord St and pick a restaurant for dinner (there are tons to choose from, and most range from good to very very good)
The ocean.
Like An Arky I’m close enough to DC that it’s easy to spend a day there, but I live near Dulles airport and one of my favorite places to take people is the National Air & Space Museum Steven F. Udvar-Hazy Center. The Enola Gay’s in there! And a Concorde! And an SR-71! 
And the Enterprise That’s the crown jewel of the Udvar-Hazy collection IMHO. Getting up close and personal with a space shuttle is amazing. Most people don’t understand how HUGE they are.
Enjoy,
Steven
NASA - the Johnson Space Center here in Houston.
Wow…this happened to me like six months ago. Sadly, there’s nothing really SUPER amazing in my area. I took her to the coast and we went to a hokey Ripley’s Believe it or Not! and also a very very VERY sad and pathetic “Underwater Gardens”. After that we drove three hours to the nearest Mt St Helens visitor’s center but that was about it.
Oh! I also took her to see the Spruce Goose. We were running low on time though so instead of paying $15 each to spend five minutes to look at the plane, I brought her into the souvenir shop where you can see the behemoth JUST fine and all for free.
The Alamo Drafthouse, preferably to a special screening. It’s dinner, drinks and a movie all rolled up into one, but more than that, it just has a very Austin vibe to it and is renowned internationally. And, of course, it’s really, really fun.
We’d start at Grant Grove to see the General Grant tree. Once that was out of the way, we’d head out and either hike to Tokopah Falls or Big Baldy.
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If my best friend from out of the area was particularly hardy, we’d summit Mt. Silliman.
It would depend on what they like. If they were into nature/history types of things we could take a day trip to Mammoth Cave. If they liked amusement parks, we could go to Holiday World, which is fairly small but has the world’s best wooden roller coaster and a large water park (unlimited soft drinks also included in the admission price). If they were just interested in eating, I would take them to one of the mutton barbecue restaurants in town.
When my brother or I bring a girlfriend or boyfriend (respectively) to meet our dad in our hometown of Milwaukee, we always take them to the Milwaukee Public Museum. The Old European Village there is delightfully creepy: a dark labyrinth of faux cottages with windows that reveal the interiors of traditional homes from every country of Europe, complete with mannequins in traditional dress going about their everyday lives. I dimly remember childhood dreams/nightmares that took place there.
Here in Cambridge (Massachusetts), I usually take visitors to Walden Pond. It’s just a short drive away in Concord, a lovely setting, a short hike around the pond, and provides that “you’re in a place of historical importance” feeling that people like to have when they visit Boston. Last year, my boyfriend and I finally visited the Harvard Museum of Natural History, and that may be my new go-to place. The collection is old and centered on stuffed specimens that are showing their seams, and the museum itself looks like it hasn’t changed since the early part of the century. It’s like a museum of what natural history museums used to look like - a meta-museum!
If they like beer (and seem unlikely to be offended by the concept), I’d take them to the beer church. I’d also take them to the Carnegie Museum of Natural History to see the dinosaurs.
Oh yes, the Streets of Old Milwaukee exhibit is wonderful. It seems like everyone who lives here has fond memories of it from their childhood. That and the secret rattlesnake button in the bison hunt diorama. 
Depends on how long they have to visit.
1-4 days: Dallas/Fort Worth, Kansas City, or Austin
5 days-1 week: New Mexico or Colorado
1-2 weeks: Yellowstone/Grand Tetons or Grand Canyon/Las Vegas or the Coastal Southeast
I live in Oklahoma City.
Can you narrow that down a bit? Any particular ocean? 
In the Cleveland OH area, I take them to the Holden Arboretum, then to Jacobs Field (or whatever they’re formally calling it now).
My workplace…aka the school.
Seriously, Lloydminster, Canada is boring boring boring. We have an small artificial lake, and the oil patch. Whoop de doo. Might as well go look at a puddle.
Don’t drive east, that way lies the wastelands (Nebraska or Kansas):D.
Serious answer, same here, usually I take them to Estes Park or Rocky Mountain Nat’l park. I like hanging out in Estes during the off-peak season.