You have to visit the Pratt Street Ale House - they have tons of awesome British ales. Steve Jones, the brewer, has been there for over a decade and makes some amazing stuff!
It's practically across the street from Camden Yards.http://www.prattstreetalehouse.com/
Also near the Inner Harbor, is the Alewife. It's relatively new, but they're definately on the map with a ton of awesome beers on draft.http://alewifebaltimore.com/
I work across the street from the Aquarium and a good place that we recently walk to is to is the Linenkugel Beer Garden - nice open setting wth an outdoor beer garden as well as an indoor section that's primarily all glass with huge garage door type glass windows that they open up when the weather's nice. http://www.leiniebeergarden.com/
Have fun in Baltimore, it's a pretty decent beer town!