For my 4-year-old's birthday party, I built a wind tunnel for her and her friends to "test" the flight characteristics of stuffed animals, balls, toys, and anything else that fit into the 4-inch diameter tube. Powered by an air pump (normally used for a bouncy house), the tunnel was oriented vertically so items could be launched towards the sky. See the video below for some of the tests:
I considered the wind tunnel a total success as it kept a herd of 3-6-year-olds occupied, let them play and discover what items would fly, and simultaneously was interesting enough that all the parents wanted to play, too.
I built the project organically with no more of a plan then wanting to have kids play with a stream of fast moving air. If you don't want to follow along to see what I did specifically for my setup, here's the summary:
Consider cardboard concrete forms for projects that require 12-inch diameter tubes
Desktop fans that fit inside such a tube can't move enough air to be fun
Not only can an air pump from a bouncy air move enough air to fly all sorts of things, but it produces a collimated stream of air; a leaf blower would probably work just as well.