The only uniqueness you'll get out of an older Mac is the crunchy 8 bit 11/22KHz sample rate. They're not unique in the way an Atari POKEY or Commodore SID chip is.
Super Studio Session will do something like that, though.
If you want to try something odd, get a Digidesign Audiomedia card and a copy of Turbosynth. It's a modular synth program that runs on the DSP on the Audiomedia, kind of like the Nord Modular except onboard. That said, it's probably nothing you couldn't achieve on newer hardware with less hassle.
Also: viewtopic.php?t=1664