Challenging programming topics

After a boring day spent programming Enterprise Java™, I like nothing better than to come home and program some more.

Currently I'm implementing the popular cryptographic algorithms in Go.

What programming topics do you use to challenge yourself?

>>5
Not impressive either. Having them done in assembly language would barely touch the "challenge" level.

I suppose this does not hold for RSA, though. In C it would be challenging enough (without depending on GMP).