If there are Ripley's museums in your area I would recommend those. They are packed with a variety of displays and some of it's worthy of history museums. I visited one in Las Vegas (no longer there it's been moved) that was fascinating.
The Mighty Eighth Air Force in Savannah is cool. You did not specify where you are or what you like. In St. Petersburg Florida, there is the Salvador Dali. In D.C. is the Smithsonian, In St. Louis Missouri, is the Art Museum, Chicago has the Museum of Natural History, In New York City is the MOMA , plus several more. Pensacola, Florida-NAVY (Air)__Dayton, Ohio- USAF,_ Arlington, Virginia- ARMY. I can't remember the exact name of some, but EVERYONE in that particular area knows about them. Just for your information, there is a small museum called the ap<x>pleton in Ocala, Florida, right near Silver Springs! Good luck.
I think it depends where you live. Why not google museums near where you are?
Te Smithsonian is a big Museum. You can spend all three days there.