Getting Back Into (Mathematical) Shape

I've recently been reading a fascinating article entitled, The Mathematician's Lament. By coincidence, I also had an article about solving Boltzman's equation up in another tab. Both of them have started me thinking: back in grad school, I had to do a reasonable amount of math on a regular basis. Calculus, algebra, geometry. Nothing really difficult, but it kept me in practice.

Now, though, I don't use much of it. A lot of statistics and some discrete math, but not much else. I'd like to keep in practice, but it's tough to find a place to get interesting puzzles to play at. Googling mostly turns up stuff aimed at kids: the "make math fun!" dreck that the Lament laments. I have all my old math textbooks (especially discrete math)  Anyone have any suggestions? (feel free to pass along the link to this post to anyone who might)