Boston is hella fun, man. My lady lived in boston for a few years and I used to go out there every couple months. I always enjoyed (some already mentioned)
1) Fenway is nice, but fuck ticket prices and lines for beer. Go sit at one of the sports bars adjacent to it, its just as much fun and youve got a full bar right there.
2)During the day head over to cambridge, I always hit natural history museums when I go places, and harvard has a nice little one. Plus the square is nice and there are some cool little shops and decent resteraunts. Theres a little bar there called grendells den that a spent many an hour drinking at. They got good nachos, which are hard to come by in boston for some reason.
3)If you're a shoe head at all hit Bodega, its near backbay, its a shoe store an elevator ride down from a fake bodega. They have bouncers and check your bags and shit, its super pretentios but worth checking out.
4)The north end is Bostons little italy, there are some cool resteraunts and if youre with a broad go get pastry at Michaels. You'll see the line. This is also a kinda nice walk from Faniall hall... or maybe its a nice walk and a short cab ride, i forget. I think faniall hall is shitty and touristy, but you might as well check it out.
5)The best classic italian food is at Giacamo's, but there's two of them one in the north end and one in backbay. go to the one in backbay, shorter lines and better food. Get the lobster ravioli. (Theyre both cash only)
6)There's a GREAT asian fusion place called Pho Republique. Go there, sit at the bar and order the PuPu platter. You won't be dissapointed and you and someone else will be totally full. I'm not normally a fan of asian fusion, nor hipster spots, but this place pulls it off.
7)Drink heavily. I was drinking pretty hard when I used to travel back there, lemme know what neighborhood you're staying in and I can probably recomemnd a good dive bar.