WWDDGD events are planned and run by the individual shops that host them, so they can vary dramatically. As already mentioned, WotC provides an adventure and characters and some giveaways, but at the end of the day it's up to the shop and the local D&D players who work with the shop to make the day into whatever they want it to be.
Playing D&D (the RPG) is usually the centerpiece of an event, but there may be other types of play (Three-Dragon Ante; Inn-Fighting; DDM (this year WWDDGD is the same weekend as the release of Desert of Desolation)), as well as general hanging around and having a good time with other D&D players.