Andrew,
Sounds like a great trip you are going to be apart of. I 2nd the motion that you CAN'T miss Colorado. I mean the state has some of the best Microbreweries anywhere, music festival galore in the mountains all summer long a few are HUGE, Amazing mountain scenery..There are many states worthy of a rush through if need be, but Colorado should not be one of them.
Memphis and Charleston are two deep south recommendations- Nashville for sure and Virginia is beautiful. Might skip fast through Texas with the exception of Austin and a drive through Dallas. Santa Fe is must do...you will love it for about a day...I might slide through northern Fla and bypass Atlanta and take in Savanna. Too bad you will miss other great states in the north.
The US is big...Enjoy what you can and make a plan to come back and do the rest!