Install LFS
Install a minimal working system (from boot to dash)
Local webserver/wiki is a good one (will help you keep up with your notes, and can be put online later if you feel like it for everyone to benifit)
Build a LiveCD (nothing handier than a good LiveCD. I say that from experiance, too)
Patch and compile your own kernel.
Write a module for the kernel

The list goes on and on. A LFS install is time consuming, but you can learn a bit about how and why things are done in a specific way, learn to use a few additional tools (readelf is one that I kicked myself for never noticing), and pick up a trick or two on how to build a chroot toolchain that can be used for other purposes. Going to a minimal system will give you an idea on how much can be done with how little, and may be fun to try and get a fully working system in under, say, 5Mb drive space. Actually, that would be a neat place to drop a web/wiki server.