'While back, Rich linked a 3D exercise which is pretty well appropriate for any age. You start with a paper circle and at the end, after a fair amount of collaboration, team-building, and discussion, you've got this sweet icosahedron. Along the way you review a couple hundred geometry concepts and their properties, tattooing down names, facts, and figures, anything that comes to your class' collective mind, anything from "isosceles trapezoid" on to "snowcone" and "taco."
My first period, I got too excited as the icosahedron grew and took too central a role in the connecting/taping process. During the second class, I didn't touch it. Instead, with ten minutes left in class — just enough time to finish if the class was motivated — I scooped some errant stack of papers off my desk and announced, "I've got your homework assignment for Memorial Day Weekend right here. I'm willing to cancel it [reluctant pause] IF you guys can make the icosahedron before the bell rings." [cue frantic team-forming, leadership-role-assuming, fun-having, and homework-canceling].
So thanks, man.
If you're dying like I am here, it's a low-stress and low-impact way to fill a thirty minute bloc without falling prey to the pro forma time wasters: games, movies, and parties. I extracted the relevant bit from a larger pdf.