I've been thinking about an afterschool class/club I could run (with help) at Cantell. I helped on a maths masterclass, which was fairly simple but the contact with the kids was amazing. Recently I found HackityHack which is cool, but it's still beta imo. It also involves writing code, with all the attendant complexity.
The best find was Scratch. I almost couldn't stop playing with this yesterday. There's a Wiki textbook project. I found a Snake game, but sadly it seemed a bit too complex to work towards over a short class (say 6 lessons).