Biking
Believe it or not, but Cambridge is one of the most bike friendly cities in the US. Remember that if you don’t use a helmet, you can get ticketed. There are many bike shops and rentals, but it is probably cheaper to get a used bike, or to get the cheapest one at Target (some 70$ and should last for a year). From 2012 also the bike share program The Hubway has been gradually expanding to Cambridge and Somerville, and it is a great way to bike occasionally or even in a daily basis. Moreover, they have special offers for Harvard students and personnel (showing a Harvard email): in 2016, that meant 50$/year pass.