When it’s hot, and you need some excitement, it’s time for water rockets.
Yes, you can buy a really cheap version for $5 and it works fine. But real geekdads show their kids how to hack one using discarded soda bottles. It will take longer to launch, but the possibilities and variations are endless.
Water rockets can be quite complex, and their physics are detailed. You can learn a lot by making them. The world record for highest altitude for a water rocket is almost 2,000 feet.
And yes, as you might expect, there are water rocket leagues and classifications for water rocket championships.
To get started with something quickly in the backyard the simplest instructions for making a water rocket from a soda bottle can be found at the fantastic Instructable site. This HOWTOON (which is also downloadable as a PDF) tells you how to make one using coat hanger, an inner tube, and of course a plastic soda bottle.
Very good advanced instructions can be found on the web too. Here is a guy experimenting with multi-stage water rockets, and extremely long models. He supplies his own plans and patterns.