What happened to the Boxster?
Since I am on a Vette kick I'd be tempted by a non-Z06 (for under $20K) manual C5. Still fast even by today's standards, reliable, easy to work on, loads of aftermarket and online support. Hard to go wrong there.
A late 4th Gen f-body would be good and I think you'd enjoy it a lot more than you expect. Basically a C5 Corvette but with 4-seats and better ride quality...Maybe like a GT version of the C5. $20K would get you a pretty nice 6-speed WS6 or SS.
SLK55 like Gurgies would hit my list too...proper AMG fun and no major German mechanical faults.
Dent's TR8 idea is a good one too, super unique and I really like the way mine drove....A British Corvette, seriously cool car.
I've been extremely happy with my XKR and owning it has really killed my desire to have anything 'more'...The 510hp SC 5.0L is mad, but it'll cruise along like a luxury car when you want it too. You'd be hard pressed to find an 07+ XKR for under $20K, especially a 5.0L, but the non supercharged XK's can be had and are a lot less complex.
Think about what you'd want to do with it....If you want to rip around locally and maybe to a track day/autocross or two through the season, then I'd focus on something like the C5, SLK55, the true sports cars on the list. If you want to take a few road trips, think about something with some GT traits....SL, Jag, even the f-body....There's no way I'd want to do a 2+ hour trip in something like a Miata, S2K, 370Z, Z3/4.