what do you like? Art museums? The Walters in Baltimore, the National Gallery and the Reynolds Center/national portrait gallery in DC are all superb and free.
Beer? Max's in Fells Point in Baltimore has 100 brews on tap; Churchkey in DC has 50.
Baseball? Camden yards in Baltimore is great.
Historic sites? Try Arlington Cemetery, Georgetown (yes, they're there if you get off M street), Capitol Hill, all in DC. Baltimore? Don't know where to begin. It is loaded.
If you have a car, civil war battlefields are all over.
Where to stay? depends on your pocketbook, how you come and what you want to see.