As others have said, Grand Canyon....I don't see how you can go to the GC and not be impressed lol.
But then:
*Yellowstone Nat'l park plus Grand Tetons (see both sides). Yellowstone Park really is pretty cool.
*A puddle jumper flight into any national wilderness area- there are hundreds- but being 100 miles from a road of any kind. I like to go to Frank Church Wilderness on the Continental divide personally- but it's more the idea of being so far away from everything but still in the U.S.
*San Juan Islands Washington to see the Orca's plus Olympic National Park- which is a true rainforest and it's pretty much 100% green. Everything, rocks roots- everywhere. Have you ever stared at something green then you look away and see a red afterimage? On a bright day you get that there.
*Alaska- Denali Nat'l Park. and Kenai Fjords. If you hunt and or fish, there are a whole bunch of other areas to recommend.
*Needles and Moab (pretty close to grand canyon). Mountain Bike in Moab if you can. Moab is sort of touristy now for bike riding, but it's still amazing.
I don't know, list kind of goes on and on

. I want to go to Area 51 too lol- just so I can say I've been there.