Cool Places in Boston/Cambridge?

My friends and I have pretty much exhausted the fun that is to be had in Harvard Sq., Faneuil Hall, and the Prudential Center. We're looking for some fun places to hang around in the Boston area where we won't necessarily have to spend a lot of money, but where we can hang out for extended periods of time and still have fun. We pretty much have the entirety of the T at our disposal and are 16 years old, if that helps..