Have you seen the water rocket plans on the TOH web page? Go to the home page and search 'water rocket' and you'll find it.
Last weekend my 12 year old daughter and I built it from the plans provided. Other than having to buy a tire valve, I had everything I needed around the shop. It only took about an hour to make and was worth every minute of it.
I have to tell you, I was a little skeptical at first as to whether or not it would actually work as stated. It works great and surpassed my wildest expectations! I think our highest flight was probably in the 75 to 100 foot range.
I would highly recommend this to both those with kids (great parent / kid project) or even to those who might just need an hour to do something fun. It is a blast - no pun intended.
The only change I made to the whole proceess was to use my compressor (turned way down!) as an air source instead of the recommended bike pump. Although on the first couple of shots the bike pump worked great also.
Be prepared to laugh uncontrollably and to get wet.