Crayon Physics from Kloonigames is a very cool serious game. I can see young kids in particular playing with this for hours. The designer said about one comment “Chris Baker wrote a great article about Crayon Physics Deluxe for Slate. He wrote that the game looks like it was designed by a third-grater. I take that as a compliment.”
If you don’t get video games this is an excellent video to watch. The kinds of puzzles kids are solving in Portal and World of Warcraft are very similar to the ones you see here. But with the interface stripped down to bare essentials you can get a sense of the brain work that is going on while players wrestle with the more complicated games.
If you want a sense of how engaging this kind of simple interface with some basic physics can be go play Linerider for a while. It only take a couple of minutes to learn it – then see if you don’t want to just PLAY.
This also reminds me a lot of And Yet It Moves – a great indie game that got some attention last year at GDC.