As someone who hasn't been able to get enough motivation to read past the first few pages of SICP, I _really_ wish someone made a TryRuby.org like thing using SICP.
Because I'm skeptical of the use of transforming SICP into a "tutorial" style. It's the kind of book that you need to read and think about.
Having said that, finding a REPL that's compatible with it can take a little bit of time. Personally I quite like GambitC which I seem to recall is fairly compatible.
Please!