After playing around with this launcher, you may want to move up to something a bit more sophisticated. Check these pages below. Good ideas from many people.
See their pages for an even more extensive list of links.

Water Rockets

Bigfoot Water Rocket: Great page, lots of good stuff here.
His launcher is a serious piece of enginering tech, the flow of ideas generous. I'm saving up for his new model "Little Foot".
Bruce Berggren's : This guy knows his water rockets. Plans, charts, graphs and spreadsheets
Water Rockets - Clifford Heath : He's got a great idea for a launcher and a stabilizer.
Water Rocket mailing list : Clifford Heath also hosts our mailing list (litserve). Lots of good discussions and ideas from around the world via e-mail. To join the mailing list (please do!), send an one-line e-mail message to "majordomo@lists.osa.com.au" containing the text (without quotes): "subscribe water-rockets myemail@host.domain". If you want info or help, a one-line mail article to the same address containing the word "help" or the words "info water-rockets" will give further assistance.

Science Fun

R. E. H. : Radical and Extreme Hobbies. A bunch of crazies, havin' fun! This ain't stamp collecting.
AGD Antics and Mayhem Page: Advanced Graphics Division at SGI. I want a job there! Liquid nitrogen, giant slingshots, huge fresnel lens', etc. Cool!
Weird Science: Bill Beaty's Homepages. One of the guys who started "cool science" on the 'net (in text even!).
SCIENCE HOBBYIST: Amateur Science subpage. All kinds of stuff for inquisitive minds.
E. M. S. work well together : cryogenics, powered flight and flies. The 'net's got something for everyone.
Flying Contraptions : Exactly.
SCIENCE IS FUN in the Lab of Shakhashiri :Not so "Exactly", but good site. For at home, small fry, fun stuff, stuff.