I live in JP. Go to Franklin Park. It's much better than the Arboretum (which is too tame for my tastes). There's The Loop, which goes along Circuit Drive (road through the park) and then around the golf course through woods and past a beautiful pond -- a lot of people walk/jog The Loop. But even better is the woods there--it really feels like wilderness. I'd go walking with you there some time if you wanted.
JP has parks on 3 sides. (Pond, Arboretum, Franklin Park)
You can take the commuter rail out to the suburbs and bike back (or inbound to another commuter rail stop). Rachel's right--get the Rubel Bikemaps, they're great. If you don't know a lot about how to ride in traffic, the Boston Bikemap (by Rubel) has great tips on the back that I still use today. Then there are maps for the North Shore, South Short, center and western MA --so you could buy one to use for your commuter rail trip.
You can bike the spine of the Blue Hills, and bike _to_ the Blue Hills (a few miles from JP). There are dirt roads with rocks where you can go mountain biking, but if you're not into that, you can bike along the paved road (if you're not scared of cars).
Brandon- def interested.
I have a group of friends who sometimes get together to explore new parts of the city but like all groups - people come and go. So it'd be great to bike with more people and see new areas!
I'm pretty familiar with downtown (i have a really fun route for introducing people to downtown) since I lived in the north end for a long time and cambridge where I live now. But I'd be interested in riding in new areas like roxbury and jp.
I bike all winter and would be up for a ride on the next sunny day (looking like next Saturday but also a brisk 33) or anytime this Spring.
Keep me posted if you're going to go for a ride-