C.O.
ETA: The Udvar-Hazy Museum in Chantilly - that's a good idea!! Cost for parking is $14 and entrance into the museum is free. Take 495 to Dulles Toll Road, exit Route 28. Or you can take the 495 to the 66 to the 28...
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A One day outing? Depends upon how much time you have...and how tired you will be from the travel and where you are staying...how you are getting around and the like...
There is a TON to do at the Inner Harbor....depending upon when you are here and you like baseball - catch an Orioles game...
There is the Spy Museum - it's NOT free and you should make reservations...
The Mall is always fun....there are new exhibits and such. If you are here Memorial Day weekend - Rolling Thunder comes through....it's a WOW thing!!
If you are staying in BWI - I would stick with the inner harbor - go to Bertha's for some sea food...
Travel over to Annapolis and tour the Naval Academy grounds....
If you tell me what you like to do - I would be able to help out more!
Have fun~!