For a two seat car Miata (see if you can get one with the removable hard top).
having said that... saw a Toyota MR2 Spyder on the road the other day... looked VERY interesting... a quick search shows they can be above budget but found one at $5K with 100K miles (which isn't didley for a Toyota).
For a sedan, if you don't mind vintage how about a Datsun 510?
Of course I'm showing my prejudiced of what I would buy in a fun car for $5K
Along those lines... maybe beater corvette
