What about doing one of the national parks?
We did South Dakota 2 years ago, and loved it. We stayed in Hill City about 30 minutes from Mt. Rushmore. If you check around there are some great cabin/motels to book. Some are cheesy. But Hill City has the best food/prices and it's not so touristy as the other city (there's really only 2 at the base of Mt. Rushmore).
Mt. Rushmore is great, Custer's State Park/Wildlife Refuge is amazing (and there is a great campground and lovely hotel inside the park), between Hill City and Rapid City is Bear Country drive thru animal park which we loved! There's a lot to do there with young kids. Crystal Caverns is close by too, and Crazy Horse Memorial, Black Hills and more.
http://www.travelsd.com/placestogo/rushmore/
You can drive or fly. I drove (19 hours straight...and there's nothing in Nebraska, so have a DVD player for the kids).
Then there's the Grand Canyon or Carlsbad Caverns or Denver's Rocky Mountains... Pikes Peak is beautiful. They're young, so wait get the parks in before they start ordering cruises and camps out of state (I have teens now... spoiled teens).