Get a 1995 Honda Civic VX hatchback. It has a 16v, 1.5L engine that gets 48 mpg in the city, and 55 mpg on the highway. You can easily find them for $5,500-$6,000.
Edit: spend some extra cash on some stiffer, lower shocks and some 15" wheels w/ 205/50-15 tires and you'll get go-cart handling, nearly 1g in lateral grip, and still over 50mpg on the highway.
