The National Parks are just extraordinary and they are not expensive if camping. If you can plan to stay for a few days all the better because most parks are huge and it is just extraordinary to enjoy the American wilderness. The bears will be your visitors for real. Some parks are just like paradise, alike Yosemite, and the facilities for camping are usually really good.
The Grand Canyon is a must of course! It is a big country full of treasures to discover and enjoy.
Los Angeles has some great museums to explore, alot of which are very close to each other. Two greats one are the Natural History Museum and the California Science Center. The Griffith Observatory is a good place to go for a few hours. It has an amazing view of the entire city, best place to view the Hollywood sign and has a good little hiking trail.
Detroit is a fantastic city to watch sports with Ice Hockey, American Football, Basketball and Baseball. Great museums and history for car lovers.