Depends on where you are traveling and how much time and money you have, but in my own opinion these are some highlights if you happen to be near by. And there are many more top attractions out there that I just haven't been to yet.

Crater Lake (Oregon)--almost looks fake it's so pretty
Rockefeller Center Observation Tower (NYC)
Washington Monument (DC)--take the elevator to the top
The World of Coca-Cola (Atlanta)
Charleston (SC)--Rainbow Row, history, seafood
Golden Gate Bridge (San Fran)--walk across it, take a coat
Pike Place Market (Seattle)--fish throwing
Yellowstone National Park
Road to Hana (Maui)
Rocky Mtn National Park--snow in July is gorgeous