Remember those marble run toys, where you put track segments together in various ways to create interesting paths for the marble? Cuboro Webkit 2.0 is a fun web version of those toys. I’ve only played with it a little so far, but I can see kids (and adults!) spending hours exploring the possibilities. You can even create an account (for free) and save/share your tracks.
I recommend looking at some of the samples in the Track Gallery to get inspiration and/or if you need help figuring out how it works.