Have you gone west? We went to San Diego and
Disneyland a few years back and had a marvelous time. I don't know where you are but San Antonio and the Hill Country are always fun if you don't want to go far.
I'd second this. I love San Antonio, and I grew up in Houston. Both have tons to do, and since the cost of living is much less than there than most of the US, you can do quite a lot for a not unreasonable amount of money.
Last year I had a little mini road trip planned, but do to health issues we had to cancel. I was so disappointed. Thinking about trying again this year. Natchez Trace down to New Orleans, over to Galveston, up to Houston, then over to San Antonio. We were going to hit the zoo in New Orleans which I really like, the Mardi Gra Muesem, do some volunteer work, check out the French Quarter, and eat Cajun food and snow balls. Then do the tree sculpture tour in Galveston, and go to Moody Gardens. Then do NASA in Houston. Then the river walk, the Mexican market, the water park, maybe a tube ride down the river as well in San Antonio. There are lots of other things to see and do in all these cities as well.
I also think the desert in general out west is great for kids. We went when I was about 10 for a three week road trip. Did the Grand Canyon, went sand sledding in white sands, saw the painted desert, visited the art galleries in Sedona, saw Carlsbad Caverns, the cliff dwellings, the Alamo, Rock climbing at City of Rocks, and went to Mexico. I remember it was just surreal to see the desert spread out before me. Even though we didn't do any roller coasters or anything it was an awesome trip. I think every kid should see the Grand Canyon and go sledding in the sand before they grow up. Amazing experiences. I've gone back a number of times as an adult. It's still cool, but just not the same.
A trip on my wish list right now is Steam Boat Springs, Colorado. They have amazing natural springs, an indoor ice skating rink that doubles as a bumper car rink on ice, snow tubing, snow mobiles, skiing, and a number of other winter activities. This will be our next big trip.
As far as other cities go.
St. Louis is a great place to visit with kids. Many of their museums and parks are free.
Washington DC of course.
Gatlinburg or Branson, but I find them both very touristy.
Charleston SC is nice as well.
It really all depends on what type of things you are interested in doing.