I second the recommentation for the Eudyptula Challenge although the "rule" about not asking/posting for help is probably why I have been on Challenge 5 for about a year.
* write a device driver for a real or virtual device; hook into the various kernel virtual filesystems that allow userspace interaction.
* write a small network stack (for example a raw protocol on top of ethernet).
* add a scheduling algorithm or mess with an existing one; analyze its performance impact.
Lots of new-to-me tools in that list were interesting. Back in my day, we had to use a serial port console, and if we were fancy we would set it up so that you could remotely cycle the computer's power when it hung up (uphill both ways in the snow).
I think I'd add using a serial-based terminal to that list too. Embedded hardware can be infuriating, but fun.
I've sent a few emails about joining the challenge around 2 months ago (after a multi year break) but have yet to receive even the automated response..