An opinion or three.
First, I’d say not the REPL. The REPL is fine for one-liners, but a pain to use for even modest multi-line functions. Ammonite is better, but I still think it’s not a good way to get a real sense of Scala.
Something in the general line of scalafiddle.io would be better as a Very First Step – something that allows you to try out real functions quickly and easily. That should be embedded front and center in the website. Better yet if there is an easily-accessible and well-indexed archive of examples you can play with.
I would love to say Scala Exercises, but can’t quite – while I think it’s not bad for the Scala beginner, it does presume some familiarity with programming. I love the tech, though, and I suspect a more novice-programmer-oriented chapter based on it would have real potential.
While I like putting an interactive fiddle tool in front, to make it very easy to mess around, I agree with @tpolecat that pushing people towards sbt as quickly as possible is for the best: we don’t want folks leaning too hard on crutches that will inevitably get pulled out from under them. I think the key initiatives to make this work are in motion – the combination of the Platform and sbt new seem critical – but folks should be constantly testing the new-user experience. It should ideally be no harder than "install this package, and then say sbt new" in order to get a valid (if generic) skeleton project, ready to use with your choice of editor. (Does sbt have an easy-to-use installer?)
Speaking of editors: folks tend to ignore the Scala-IDE, but I think that’s a strategic mistake. Yes, many people are focused on IDEA, but a lot of the engineers coming out of the Java community are used to Eclipse. Making it as easy as possible for them to keep using the tool they know would probably aid that transition, so it may be worth including the Eclipse plugin in the Platform. Give it some thought, anyway.