A GeekNic is, in essence, a Geek-ified Picnic.
Involving food & drink, sunshine and the community, they can be a great way to get everyone outside and to enjoy the natural surroundings.
Originally developed by the PDPC, (the organisation that runs freenode), the GeekNic is fairly well known throughout FOSS groups.
They are designed to bring together a community in the sunshine, to get people talking off IRC and to be have fun!
GeekNics should be "Family Friendly" Events, whereby families can bring children of all ages and relax - Of course, there are always going to be times when they aren't, but it's an idea to make them "for everyone".
The original GeekNic website is available at : http://geeknic.org