Depends on your idea of fun, here's some ideas:
Small / light RWD:
Nissan 200SX S14
Nissan Silvia S15
Small / light RWD but will get idiots calling you a hairdresser/homosexual
Suzuki Cappuccino
Mazda MX5
BMW Z3 (not sure if these are down to 5 grand yet though)
Sports Saloons that may cost a bit too much to run
Subaru Impreza WRX or STI
Subaru Legacy B4 RSK <-- for people who want a Scooby but want it well built with a nice interior and without the chav image
Mitsubishi Evo
Nissan Skyline (ok there is no MAY cost too much to run here!)
BMW (most of them, obviously not the 316 or the X5)
Toyota Chaser Tourer V
Hot hatches
Mk2 VW Golf GTI
Mk3 VW Golf VR6
Renault Clio 182
Ford Sierra Cosworth (I guess this could also go in the sports saloons section)
Toyota Glanza V
Cars that look good on paper but are going to cost you too much to run due to unreliability
Nissan Pulsar GTi-R
Mazda RX-7
FD Mazda RX-7 <-- I'm singling this one out as it's a particularly unreliable version
Big lardy tourers that look awesome and are cheap but will cost a fortune to run
Nissan 300ZX / Fairlady-Z
Mitsubishi 3000GT / GTO
Fun cars that I'd like to own but are overpriced for whatever reason
Toyota Sprinter Trueno GT-APEX / Corolla Levin / AE86 (overpriced thanks to Initial D)
Mazda AZ-1 (rare)
Honda NSX (rare)
Oddball Japanese imports that I thought I'd throw in there anyway
Nissan Cefiro SE-T (Skyline GTS-T in the bodyshell of a boring executive cruiser. If you want AWD get the SE-4 variant but good luck with that, last time I checked there were only 3 for sale in the whole of Japan, now it's only two because I bought one of them. There's a nice one for sale at the moment that's had a Skyline GTR conversion done though)
Subaru Vivio RX-R (it's the size of a Smart car, has a 660cc supercharged engine
Toyota Sera ( <3 gullwing doors )
Obviously not everything in that list is a recommendation, or maybe they are it depends what you want, if it's a project and a weekend car then the less reliable ones may not bother you so much. You can probably tell I'm generally biassed towards Japanese imports though
