This site here shows you how much fun you can have with putty and some canned air. Basically shoot the liquid from the canned air into a putty ball, as it expands into gas, the putty ball grows, then pops.
Another link from Cottonbuds, this would be a great project to make for the kids. This site shows you how to make your own inflatable bladders. What's an inflatable bladder you might ask, "You can make inflatable bladders for pool toys, dolls, balls or even kites and it's really cheap." More than just a bag you blow up and tie off, this one has a valve, you use a soldering iron to put it together, and the site has instructions to do your own minor repair work.
This site shows you how to make your own light saber. Very good, detailed instructions. Judging from the pictures, this actually makes a very professional looking light saber. Edmunds Scientific has a kit you can buy to make your own light saber as well. According to the description, it uses neon lighting, and when you touch the contact probes on the handle, the light actually flows up the blade - Very Cool!
A bit old, but extremely cool. These guys built "Augmented Reality Quake", basically, put on the backpack and goggles, run around the campus, and see/kill the monsters running around with you and other players. Extremely expensive to build since they had to use very precise tracking equipment, but this would be a great experience.
Using an ordinary match, some foil and a launch pad made from a paper clip, you can easily make your own "matchstick rocket". Here's another all text site with instructions. The instructions on the text site vary a bit, but he has also posted his best distances (44' 8") and experiments into "fine tuning" his rocket.
If you enjoy the climbing walls you use at the gym, here's a great site for you. This site not only gives you the basics to building a climbing wall, but at the bottom, has links to many other resources and more detailed information.
If you're into paintball, here's a great project for you. This site shows you how to make an air cannon, which is perfect to load up with paintballs for the ultimate paintball battlefield weapon. Other suggested uses from the site are loading it with confetti or lofting chemical glow sticks on parachutes for a safe (safer than gunpowder based fireworks anyway) fireworks show.
The CPL regularly hosts tournaments with prizes of up to $100K. Not too shabby for playing games. There was an article on Kuro5hin not too long ago describing the downsides of professional gaming, so it still may have a ways to go. Also an article on BBC talking about Korea being the place to be for professional gaming these days.
Sam's Laser FAQ provides all the information you need to get started with lasers. The site is a little tricky to at first to find the info you're looking for, until you stumble upon the Expanded Table of Contents. The site is very text heavy (many of the diagrams and schematics are done in ASCII), but if you've been wanting to build your own laser, this is the place to start.