i live only a few miles south of boston so I visit a ton. One of the things I never get bored of is just taking a stroll through the commons and the garden, walking around newbury street,Fanuel Hall theres usually some good street performances going on. As for fire and ice i'm not a fan at all, ive been there 5 times now, and while the idea is cool ,choose your own ingredients and sauce, the food is usually cooked very poorly and the price is over priced.
If you get the time id definitely recommend going to the aquarium or museum of science. Sox games are a blast too. how long will you be up here ? and around when, if its soon make sure to get the schedule for the bruins game, the bars can get very rowdy.
OH, one thing i almost forgot, in cambridge you can take a tour of the Sam Adams brewing facility, if yours a beer fan its really interesting to get an inside look into the beer world.