If you're just wanting to play around with coding you could try one of the many 80's-micro emulators (C64, Atari ST, Spectrum) that run on Mac and are also supported on the GP.
I'm fiddling around getting STOS (Atari ST Basic) running on my Mac, I figure I'll be able to save a completed disk image which CaStaway should (fingers crossed) run. I'm also intending to try and sort out Sega SC3000 basic coding if I can get that to work, I used to have one and it was a fun wee machine.
I'm sure you could get a decent wee Spectrum game running and save it as a tape image.
If you're an old 80's coder it could be way to get back into it, and if you're new to coding there's not many easier ways to code than Basic. And there's still lots of non-GP folk out there coding away on the 'old' machines and releasing the results to the public so there's lots of support.
'Course, the truly hard-core coder would be able to code Atari 2600. 'Cept you'd need an engineering degree!