One of the many cool things that NASA has on the web is NASA Quest, which offers K-12 classrooms (and others) a variety of mission-based interactions and activities for learning and exploration. A cool upcoming activity is the HiRISE Image Targeting Challenge, which gives participants a chance to help choose targets for the high-resolution imaging system on the Mars Reconnaissance Orbiter (MRO). This one will run from September through December 2007 for grades 5-14.
As an example of something that's already available on line, I tried one of the Personal Satellite Assistant (PSA) activities (separate from the challenges), which demonstrates forces and motion in a 2-D microgravity environment using thrusts applied in up/down and left/right directions to maneuver the tiny spacecraft. This is good basic training for understanding thrusters in Orbiter (not to mention physics, and real space flight). There are many other activities to check out at the NASA Quest web site, and they include materials for teachers wishing to use these activities in their classrooms.