My daughter was at the National Aquarium this past Sunday. Parked in the Harbor parking garage for $20 (it is $15 I believe if you present a admission ticket stub). Very close to the aquarium, 3-5 minute walk. You can take the Baltimore Metro subway from Owings Mills metro station and get off at Charles Center. $1.80/one way or $4.20 for a day pass. About a 15-20 minute walk from Charles Center station. We take the subway when we go to Orioles games. It's very convenient. It is a one hour drive from Gettysburg to the Owings Mills metro station.
Mark