Heard a few suggestions of the southwest and I would personally agree with spending a day or two here in AZ. If your into history, this IS the old west, and many parts of that still exist. Also my town is home to one of the best boneyards, Davis-Monthan, and its very inexpensive to get a tour of it, as well as the air museum. Also a nice thing is gas prices here are the lowest in the country ( ;)) Our mountains are 8000+ ft and are just the little foothills to the Rockies (another good place to go) and the amount of available camping here is great. City life isn't the most exciting but what we have in nature and just originality is pretty nice.
Grand canyon and northern AZ are very nice this time of year, and having been to Colorado in summer I would also highly suggest there. I'm sure you'll be able to find something that best suits what you want to do no matter where you go :D
Tombstone is down by tucson also, would recommend going there as well