I think we should only be recommending MIT Scheme. Everything else has got too much going on and can become distracting, for the purposes of education.
I don't think it is necessary to avoid Racket since "The language #lang sicp provides you with a version of R5RS (the fifth revision of Scheme) changed slightly in order for programs in SICP to run as is." DrScheme is great for this and probably the easiest way to start.
Everything you can do with Scheme you can also do with Snap! with a visual blocks programming user interface. Functional programming, recursion, higher order functions, closures, continuations, macros, metaprogramming, all described by Brian Harvey's curriculum, the Beauty and Joy of Computing:
Brian calls SICP the actual best computer science course ever invented, which is quite a complement from the guy who wrote the second best computer science course ever invented. ;)
He has a lot of interesting thoughs about AI in education.
>I admit that my slogan "Snap! is Scheme disguised as Scratch" would sort of push in the direction of hygienic macros. But historically we built Snap! more with the idea of Logo disguised as Scratch. It was just when we added lambda that we started thinking more in Scheme terms.