Last summer I took a road-trip across the USA, from San Francisco to Miami and everywhere in between. It was often the areas I didn't have in my original plans that ended up being the most enjoyable, such as Austin in Texas and Nashville in Tennessee. This is not to say we didn't enjoy the coastline on either side, however their was something touching about meeting people who had never met a European. The gentle and friendly way they treated us and took us in will stay with me forever.
I feel everyone should pick up a bag, hire a car and embrace Route 66 with a set of great classic songs and a large amount of tinned food! If you want any inspiration, there is a great article here http://www.camelsandchocolate.com/2012/09/denver-attractions/ which is definitely worth a read.
If you like to party, all I'm going to say is YOU HAVE TO VISIT NEW ORLEANS! Honestly, New Orleans makes Ibiza look like a grandma having a nap!