You could try looking up the tasks of the Eudyptula Challenge.

More than half the value was in sending mails and getting feedback, but it's still a great introduction that gradually gets you used to more and more complicated kernel dev

There are solutions lying around on the internet for the various tasks, but I'd advise not looking. The kernel is sparsely documented (some areas, rigorously undocumented), so a bit of experience searching and figuring out how to get the information on your own is genuinely valuable