What to do in Boston when all you have is a day and a half?

I came to see my friend in Boston for the weekend. Unfortunately, he lives somewhat away from the main attractions of Boston (Allston). If all I have is less than a day and a half, and I am on a budget, what is the coolest thing you recommend we do or go to in Boston, and what is the fastest way to get there from Allston (on the bus 57 route, not driving).