Probably not as much fun for a bachelor party, but my wife and I were there for something like 2 days. We enjoyed the Freedom Trail - literally a line you can follow, around the city, to all of the great historic stuff. A lot of fun if you are into historical stuff (assuming not for this). Another thing we enjoyed is this -
http://www.faneuilhallmarketplace.com/ a lot of activity, even late at night outside with street performers, etc.
I'll post anything else I remember. Oh yeha, it costs something like $10 - $15/person, but also do the tour of Fenway Park.