Miata / MX-5 or an E36, as suggested. I doubt you could get anything that's as much outright fun as the Miata, but the E36 combines practicality, fun and performance very, very well. It's a BMW, after all. Keeping the Miata running should be a good bit cheaper, though, from what I know.
The 300ZX might be a good choice as well, have heard a lot of good things about them. I'd probably go for the NA version, not as powerful, but less turbos to break down. Cars I wouldn't go with are the S13 (180SX) and the RX-7. Not a fan of rotaries and finding a good 180SX for a good price seems to get harder every day. At least around here. Guess they're a little hyped up.
/edit: Some good cars listed by Leonidae@MFT, in my opinion... Mercedes, BMWs and Porsches can be a bit costly to maintain, though. Gotta love a Z3, anyways.