Teaching Kids To Do Cool Computer Stuff?

My youngest son’s teacher found out I work at MS, so now he expects me to come into his class to teach them cool stuff. I’m happy to do that, but now I’m worried about putting together a cool 1-4 hours of stuff to do, e.g.

• Cool PowerPoint stuff (wowing their classmates)
• Cool Excel stuff (doing their math homework)
• Cool Web stuff (building their own web site)
• Cool Programming stuff (writing their own PC/XBox 360 games)
• Cool Hardware stuff (programming a robot)

Obviously, I’m not the first person to want to teach kids how to do cool stuff w/ their computers. Can someone point me at some activities for a classroom for of kids to do in these (or other) categories? Thanks!

P.S. My son is a 6th grader, making his class 11 and 12 year olds.