For 20k you could get a used Fiat 500e AND an Autozam AZ-1 in ok condition.
One for the commute, one for fun
For new cars: the Prius C starts at $20.5k MSRP, I'd bet you could get that under $20k
For used, a Honda Insight (first gen) will get you 60mpg all day with a bypassed hybrid battery.