You could essentially do anything. Heres some of what I do:
-Visit my friends' apartments and dorms just to hang out, watch movies, pig out on pizza, etc.
-Always go to the BU hockey games on Friday or Saturday night to cheer on my Terriers (who let me down big time this season)
-Go to a nightclub if I can find a girl who would like to go
-Play some pool or go bowling
-Go visit another college campus to see if any particularly fun activities are going on. Harvard, Northeastern, MIT, Emerson, and Berkeley are all a very quick commute
I'm sure there are a million more things if you don't feel like going to a party on a weekend.