Sorry I got back to this thread late. I live in New York City, so you probably have plenty of ideas where to travel to there. But my wife and I went to all 50 states and all 10 Canadian provinces in the first 5 years of our marriage, so we know how to cover ground.
I don’t know what your vacation-time situation is like, but we managed to do most of it in a trio of two-week trips. On one trip, we headed west to Los Angeles on I-40 (slight detour to hit Las Vegas), then south to San Diego and back east on I-10 until New Orleans, then trending back north through the Deep South states until we hit I-95 to get ho,e. Trip # 2 took us west on I-70 until it ends at I-15, that north into Idaho (which is one of those states that easily falls between the cracks), turned east, went through Yellowstone National Park, followed US-16 across Wyoming and into the Black Hills, hooked up with I-90, went south on I-29 to pick up I-80 near Omaha, which took us the rest of the way home (with a detour to Indianapolis). Trip # 3 was across I-94 west to Montana, then northward until Edmonton, further west to Banff and Jasper, south to the Trans-Canada Highway, east until Niagara Falls. These three “strips” can probably serve your purposes well also.
Here (in no particular order) are some specific non-obvious things to try not to miss - I’m not bothering to mention the more iconic, well-known attractions, you probably have them on your agenda anyway:
Scheer’s Lumberjack Shows in Wisconsin
Tommy Bartlett’s Show, also in Wisconsin
American Computer Museum in Montana
City Museum in St. Louis
Black Hills Reptile Gardens in South Dakota, right near Mount Rushmore. I’d recommend going soon, while your child is small, because there are few things cuter than a little kid riding a giant turtle - but only while that kid is little enough.
Cole Land Transportation Museum in Maine
Squirrel Cage jail in Iowa (just across the river from Omaha, we covered Nebraska and Iowa in a single day)
National Music Museum in South Dakota
I’d also recommend the following:
In Idaho, if you want to do something relating to potatoes (being somewhat iconic to the state), try the Idaho Potato Museum, in Blackfoot.
North Dakota is not the most interesting place in the world, but you might enjoy (as we did) a tour of the Pilot Training facilities at the University of North Dakota in Grand Forks.
Finally, I cannot recommend Arches National Park in Utah enough. I think it’s the most beautiful of all the national parks I’ve been to.
I’m sure there’s a ton of stuff I’ve left out. I love travelling the USA.