As a civil engineering major, I am a huge fan of the Bridge activity, the winner of a Physics Game Jam organized by OLPC.
The kids at Ecole Shalom have no trouble with the Physics activity, but Bridge requires much more precise mouse motions and clicks. I showed it to a few students and they enjoyed the mechanics and sound effects, but we didn't get any bridges built.
Last week, I got the idea to change up the game. As you play, flowers rise up and you can use them as supports for your bridge. I connected the flowers' growth to a solar sensor. Put your computer in the sun, and the flowers will grow and support the bridge. Watch out for clouds and saboteur classmates, because without sunlight those flowers will wilt away! ALSO to stop kids from cheating by connecting the sensor leads, the maximum flower height is too high for a train to climb.
See images from our classes