Angela, I am not a kid anymore but I sure got a thrill from standing in four states at once! That has been on my list of "things to do before I die" for a long time and I'm glad I was able to do it.
Haven't been as well-traveled around the US as I would like, but of what I've seen, I'd say:
Grand Canyon
Mesa Verde
Silverton, CO
Bryce Canyon N.P.
Arches N.P.
Zion N.P.
San Antonio missions
Texas Hill Country
Washington D.C. monuments
Florida Keys and Key West
St. Augustine, FL
Smokey Mountains and Blue Ridge Parkway
Cheers,
Michelle P.