Steven Strogatz did a series of blog posts at NY Times going through a variety of math concepts from elementary school to higher levels. (They are presented in descending date order, so you may want to start at the end of page 2 and work your way backwards.) Much of the information will be old hat to LWers, but it is often presented in novel ways (to me, at least).

Specifically related to this post, the visual proof of the Pythagorean theorem appears in the post Square Dancing.