I drive a rebuilt metro from the early 90s. It was extremely inexpensive, is easy and inexpensive to fix (it's only needed a new water pump and a clutch adjustment in the 5 years I've had it), and gets 30 mpg. Furthermore, it's just as environmentally friendly as a lot of the more expensive options, more so than the new "hybrid suv" monstrosities, because it's about maintaining something old rather than creating something new to be thrown away.
A motorcycle or scooter is fun, but it's not reliable year round transport in new england.
If you have a little more to spend, look for an old Toyota Echo--it's a comfortable, fuel efficient ride, very well designed, I think.