At the San Mateo Fair Grounds (http://www.makerfaire.com/) its for those who make things. Go check out the website, there's some really neat stuff for the kids (including a life size (6500 sq foot) Mousetrap game) I've heard lots of good things from those who've gone before.

Oh, and there's a craft fair of already made stuff to peruse...and stuff to buy if you are a "Maker"