The National Air and Space Museum in the Mall is so awesome they made an attic for it: the Steven F Udvar-Hazy Center in Chantilly, VA. It’s three times the size of the DC museum and sprawls through massive hangars near Washington Dulles International Airport. Highlights include the SR-71 Blackbird (the fastest jet in the world), space shuttle Discovery (fresh from the clouds after its 2011 retirement) and the Enola Gay (the B-29 that dropped the atomic bomb on Hiroshima).
Visitors can hang out in the observation tower and watch the planes take off and land at Dulles, go on a simulator (piloting a jet, taking a space walk), or catch shows at the on-site Airbus IMAX Theater. Free 90-minute tours through the collection are offered at 10:30am and 1pm daily.
If you're driving, take I-66 West to VA 267 West, then VA 28 South, then follow the signs. Parking is $15 (free after 4pm).