I went to st thomas in us virgin islands and it was fantastic. there is allot of things to do there. you can see the cruise ships. shops galore. the beaches are out of this world especially st john. it is a bit pricey though.
Gatlinburg Tennessee........the mountain veiws are spectacular,you can go hiking,canoeing,flyfishing,skiing,tubing,or you can check out all the different waterfalls,watch for bears which are everywhere head to town and shop or eat take the skylift up to top of a mountain take aride through the mountains to much to do to name it all
it's hard to say since the US is so diverse...but the best place would probably be New York City. There is a lot of stuff to see and do, basically anything you can think of. Plus if you wanted to see some nature the mountains in upstate new york are good for that (niagara falls,etc).
But if you don't like the city any of the national parks would be nice places to visit. Yellowstone, the Grand Canyon, Yosemite. OR, if you like the beach, Hawaii is definitely the best place.