As people mentioned, not much along the lines of transition around. Orchard has a 5ft mini ramp, so that's your best bet. Lots of younger people live in Sommerville, Jamaica Plain and Allston if your looking for something a bit cheaper, but still accessible on the T. Orchard's in Allston as well. More expensive living is obviously in the heart of the city, back bay, fenway, etc.
A few random spots... Ruggles basketball courts/ledges right outside the Ruggles T stop, behind the police station. Jackson Mann school in Allston, there's a really fun circular manny pad in Sommerville near the RAW store (and some more spots over there, just ask in the store), and of course Eggs which someone mentioned is across from the sheriffs office thing.