If the OP is truly concerned about his/her impact on the environment, he/she should be wary of getting a small scooter. They do get great gas milage, there's no doubt about it, but they spew an awful lot of crap into the air. The problem is that (unless you're in California, I think), they don't have catalytic converters in the exhaust stream, so you get a lot of unburned hydrocarbons in the exhaust. A lot more than your average car. This is compounded by the fact that scooters in the sizes you're looking at are usually two-stroke engined, meaning that they put out a lot more in the exhaust than their four-stroke counterparts due to, among other things, the addition of two-stroke oil to the gasoline.

This isn't to say that they aren't useful or fun - they definitely are - but one of the stated goals was lessening such effects.

Oh, and check the licensing requirements in your jurisdiction. They vary pretty widely.