A must-have site for all GeekDad’s is Bill Beaty’s Science Hobbyist site. It has tons and tons of really cool things to build with/for your kids and your own inner kid. Bill worked at Boston’s equivalent of the Exploratorium making science exhibits that appeal to kids, and he continues his hands-on approach with informative and yet simple advice for making such things as Van de Graaff generators, and so on. This is Bill here with one of his static electricity makers.
For something really simple to make — from scrap pieces of plastic mirror — check out this YouTube by Bill on how-to make a digustoscope.
You can easily spend hours on his site and videos getting ideas and inspiration.