[biot]

A tad more complicated than C64 BASIC, but: http://www.templeos.org/

Terry Davis (creator) likens it to the C64, though you're writing in CPP/ASM:

  "The vision for TempleOS, however, is a modern, 64-bit
   Commodore 64. The C64 was a non-networked, home computer
   mostly used for games. It trained my generation how to
   program because it was wide open, completely hackable. 
   The games were not multimedia works of art, but generated
   by non-artist."

[sbuttgereit]

That is awesome. I always joked with my wife that if we ever get our kid a computer early on, it wouldn't be an iPad, etc. that I'd figure out how to only give him exactly the sort of thing I had when I was a kid (a C64) and if figured that out, he'd be OK. The joke now becomes reality! :-P