You don't need a car to get around Boston for tourist purposes. The "T" (Subway/Bus system) runs decently enough to use- you can take the Silver Line "bus" to South Station and connect to the subway there. (Or you could go on the shuttlebus to the Blue Line-Airport station and take that)
You can take the subway to Fenway, Prudential, Aquarium, Chinatown, Harvard, etc... although when downtown it's usually much faster to walk the few blocks.

The T closes down at 12:30ish, but taxis are plenty. Driving & parking aren't very pleasant experiences, so don't rent a car unless you're going somewhere far.