I suggest the aquarium and possibly the 90-minute harbor cruise past many decaying facilities - and some active ones.
Go to Annapolis if you haven't been there. It's an active sailboating center, probably the largest in the US. Maryland's capitol building was serving as the US Capitol at the time the US ratified the peace treaty following the revolution - the newer part is still in use. Main Street is a bit yuppified. There's also the US Naval Academy - but I don't know how they treat non-US visitors now - there used to be no formalities to enter. All of this within 10 minutes walk.