This is a fantastic idea for a fun picnic any weekend there is some fine weather and a bit of wind.
The basic notion is to organise a picnic or BBQ with the request that those attending all bring along a kite to fly before/during/after the meal.
It's a simple idea but heaps of fun, and you get to relive all the good times you had as a kid trying to get your brown paper invention to take-off, and oohing and ahhhing at everyone else's attempts to get airborne.
Ask most people over 20 and they will not have flown a kite in years.
You can buy a kite for next to nothing from a $2 shop, or it's lots of fun and surprisingly easy to make your own flying contraption.
The simplest instructions I could find are here and they worked a treat for me. This page, and this page are also very good.
Of course there are hundreds of different kite designs and once you get the basics of kite making down it's quite easy to make almost any of them.
The Kite Zoo has a great list of basic kite types along with illustrations.
You can also find videos on how to make a kite, ranging from the pendantically detailed to this guy who made one out of two pencils and an A4 piece of paper.
It's also lots of fun if you have a few games people can play once they get the kites in the air. This website has a great list. My favourite is definitely the Altitude Sprint.