If I'm not motivated enough, I would not dig into some particular code base just for fun. Usually I try to find out, how somebody else solved a problem or designed a system, which interests me. I often investigate different approaches of different projects, to spot similarities or differences. I also watch out for approachable and readable code, which corresponds with my level of knowledge. I like in particular small projects, with simple makefiles, a clear file organization and clean code.

Thanks