Cheap and Fun things to do in Boston?

My sister is coming to visit. We are both college students so broke...
any ideas of something to do in Boston to show her the sights on a budget?
Thanks!

ok, the swan boats on Boston Public gardens cost like $2 dollars and is a nice ride. After that you can cross the street and get a drink in cheers (no lunch). You can go to boston harbor for like 1 dollars you can go to chaleston to see US connustation for free, i can go on, if you want more let me know.