+1 for the zoo. But beware that parking starts at $10 and maxes out at $20 for 3 hours or more.
Udvar-Hazy parking is now $15, but free after 4:00pm. Note that Udvar-Hazy is open until 6:30pm during the summer. But if you arrive after 4pm you might not be able to take the kids up to the "control tower" which has a great elevated view of IAD operations.