I think you're pretty much on the right track -- looking at and selecting a relatively simple driver that exists in the kernel currently, and studying that. Just look through the source code of the kernel, and select a driver source.c file that's really small, like maybe a keyboard driver
Also, if it helps at all (in case you missed it) there's a facility within the more recent kernels where you can insert your own driver/module code to be built with the kernel, and test it out.
Sasha