I have come to understand that I really enjoy learning about parses, compilers, VMs, and other things along these lines. For example, I really like implementing parsers. (I'm not sure why, though...)
I have very limited knowledge of C but it looks like the final program is simple and short enough (250 lines of code) that I'd be able to follow the full tutorial and implement it in C. I started reading it and the prose looks quite accessible, so if you're into lower-level stuff, give this a read!