The National Air & Space Museum is a must-see for all visitors to Washington, DC. Even if you're not a huge aeronautics fan, there is something for everyone here. Huge planes hang from the atrium ceiling, and a recreated Space Lab lets visitors get and up-close perspective of the insides. Individual galleries highlight aviation, aeronautics, and space travel. There's an IMAX theater (which isn't free), two fantastic gift shops, a full-service restaurant, and a McDonald's/Starbucks. You can easily spend an entire day here if your feet and back can handle it. I suggest taking a curator-led tour of part of the museum, which is really helpful in understanding the history behind some of the planes.