I went to the US twice and loved it every time. I'm certainly no expert, but here's what I enjoyed the most:
If you like rollercoasters and theme parks, I STRONGLY recommend Cedar Point, which is a theme park relatively close to Cleveland. It's probably THE best theme park in the world as far as rollercoasters are concerned, certainly the best I've been to. And if you go, ride Raptor for me, will you?
If you're staying in Western USA, Six Flags Magic Mountains is also a very good theme park (Riddler's Revenge and Scream are awesome!)
If you like TV things, the Warner Bros Studios tour in Los Angeles is really interesting. You get to see many decors from your favourite shows and it's a lot of fun.
If you like sceneries, go to the Grand Canyon, absolutely. One of the most beautiful things I've ever seen was the sunrise on the Grand Canyon. Breathtaking!
And just for the fun of it, go to Las Vegas. I'd never have thought I'd like Vegas, but in fact I adored it. The hotels along the strip are like monuments you get to visit, and the atmosphere just makes it all... magic.
So that's my 2 ct worth!
Kaethel