I went about 2 years ago with my family. If the weather is nice, you could always take a walk up&down the boardwalk. (I don't know how far along you are though...its a long walk...). There's lots of statues/monuments/etc. along it, and of course tons of restaurants and shops right across the street...Its simple, but you said you were looking for something that didn't require much activity...Usually at night, there will be live music playing on the boardwalk in several spots...
The aquarium was cool too. Its bigger than the one in Calvert, so you get to see more variety of live creatures.
Depending on where you stay, you could probably also save gas by riding those trolly/bus things. They stop at all the major attractions.
Have fun and enjoy your trip!!
