Can't add much, most of it has been said... .
Southern UT is great as already stated, and don't forget Arches NP. Moab, 4 corners, Mesa Verde and wander into So Colorado. Someone else said Colo too.
I have been several places in the US but not nearly all. Maine, Vermont and NH were gorgeous in late Sept. Went there just before I want to 'Nam,
Have not been to the South East, but did live in Houston for 4 years... also Ill, AZ, CO, & ID
One thing to consider in Colorado and Utah mountains is it is hunting season. you won't have any problems in parks, just don't wonder in the woods with out blaze orange.
If you have a mind to go to California (born & raised there) try Kings Canyon and Sequoia National Parks. A little farther north in Yosemite..... You can't imagine how big those trees are. If you go farther north there is a wonderful State Park... Calvarus, Big Trees. My wife was most inpressed with it for the peace and almost spiritual feelings there. I also took her to the Redwood Coast, but she said nothing compared to Calvarus. There is also a cave I took my kids to nearby. The other thing from Yosemite north I usually take highway 120 if I remember correctly and work my way to highway 49. This is the road through the 49er Gold Rush country. There are lots of old mining towns, Gold Rush museums, State Parks, etc... . Look for the book in a library "Ghost Towns and Mining Camps of California" It mostly follows Highway 49, but also includes some high country areas and desert ghost towns too.
The other choice is the coast. They have real waves compared to Texas
Trees, waves and crowds of people are probably the only things CA can out do TX on... .
Oh Ya, October and April/early May are the best times to visit CA. Any other time it is soooooooooooooo crowded. Nov/Dec/Jan/Feb/Mar are the sort of rainy seasons up the north coast (Santa Cruz north) other northern areas just Dec/Jan/Feb.
Let us all know, sound like you will have ablast regardless what you decide.
doc
Attention: TDR Forum Junkies 
: Natchez Trace, Blue Ridge Parkway was a very nice experience for us. If you go & like aquariums do not miss The Tenn. Aquarium at Chattanooga; 7 stories tall--ride an elevator to the top & meander down surrounded by sea life. 
