+1 on the Udvar-Hazy suggestion. The museum is great; the admission is free; open from 10-5:30 every day but Christmas; MacDonalds-operated food service is "OK"; very good IMAX movies (see "Fighter Pilot"); 50 cents for the shuttle bus ride between the airport and museum.
The shuttle leaves from curb 2E (look for the rental car shuttles and parking shuttles, and find the 2E overhead sign in the middle of that area. The first shuttle on weekdays is at 10:30am, and then every 45 minutes. The return schedule on weekdays starts at 10:45am, and then every 45 minutes. (Weekend schedule is once an hour - you can find it online.)