Go to an O's game! Always fun, tickets are usually pretty cheap, and you can bring food in to the stadium.
Stroll around the Inner Harbor. There are tons of shops and restaurants. For breakfast/brunch, check out Miss Shirley's Cafe. Otherwise, there are good steak houses along Pratt St as well.
If you've got young ones coming along, this is a great place to spend a few hours right near the inner harbor.
Beautiful park overlooking the inner harbor. Right next to the American Visionary Art Museum.
Old historic neighborhood on the water with cobblestone streets, unique shops, and great bars/restaurants
Neighborhood with a street nicknamed "The Avenue". Full of unique shops and restaurants located in old rowhomes and houses. Great restaurants include The Food Kitchen, Luigi's Deli, and Common Ground coffee shop.