Great, fun, possibly romantic, definitely cheap things to do on an October weekend in Boston?

Hmm...you could go to Arnold Arboretum in Jamaica Plain. It's an arboreal learning center (a tree museum, of sorts), part of Harvard Univ. Basically, it's a huge park with many different kinds of trees, and it's gorgeous. Even better, bundle up and take a trip to one of the Boston Harbor Islands. They shouldn't be crowded and are beautiful as well.