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?

There's this fag that sold this system i'm supposed to rewrite to the company i work at. Thing is written in delphi (for UI basically) but all the business logic is inside ms sql server stored procs. Most terribad shit I've ever seen, but this guy is laughing all the way to the bank. Why ? Because he explains everything single stupid facet of this program in such a way that it makes the powers that be think he's a genius. Ye olde BBB. This guy can talk for 8 hours straight about a stupidly simple localisation framework. When I talk about my loc framework in comparison i go, 'yeah, its a framework, gives u ability 2 different languages'. He wins, I lose.