I've recently been enjoying using the mathematical problems listed at Project Euler to learn Python. My Excel ability is better than my Python, but I think I'd still benefit from the sort of inductive learning that comes with solving a series of increasingly difficult puzzles using a particular tool.

I know Project Euler can be completed using Excel, but are there any other puzzle series similar to this or The Python Challenge specifically tailored for people trying to increase their knowledge of Excel and what it can do?

NB. I'm not looking for a "tutorial", I know there are plenty of these. And apologies if this isn't completely appropriate for programmers.SE.com - some of the folks at SuperUser suggested it was a better fit here than there!