Well, there is the aforementioned Fenway.
Right with Fenway, there is Landsdowne Street. There, you can take in the Cask and Flagon pub for some good drink and atmosphere. Then, head down the street to Axis. One of the clubs in the city that usually draws a fairly..ahem...interesting crowd. Right next to Axis is Mama Kin, Aerosmith's club. Then, at the end of the street, either hit Jillian's for some pool, or Boston Billiards is right around the corner.
Then there is Chinatown. Certainly not on par with New York's Chinatown, but still a load to see there. Right near Chinatown you can either hit the theater district, or the historcial district near the Commons.
There is the Museum of Science over near the Longfellow Bridge. Usually some fun stuff to take in there.
Harvard Square off the Red Line is a good place to hang out, with some great shopping and, well, duh Harvard.
If you continue on the Red Line, go into Porter Square to Red Bones for some great BBQ. Most definately worth the trip just for that.
Quite a bit to do in the city as you can see. Spent 8 years working in Central Square, and found my way around during that time. Also father-in-law owns a store in Davis Square (was in Harvard Sq, but just relocated due to insane rent). Good luck and have fun.
TJ