...Miata, Miata, Miata, Miata, Miata, Miata!!!
They are cheap to buy, cheap to fix, cheap to insure. Cheap fun? You bet! Thats the most important part about a "summer" car, it has to be fun, and the Miata is at the center of it all.
...But if I was to have $20K in my pocket right now and I had to buy one today, I'd be going for a 2001 or 2002 Chevrolet Camaro SS with the T-Tops and the Tremec T-56 option as well. Given that it performs nearly the same as the Ford Mustang GT500 convertable even while being down 175 BHP, its a screaming deal, as they are quite easy to live with, getting nearly 30 MPG quite easily.