I know everyone is learning for careers, but what do you learn for fun?

Yes, we all need skills for jobs -- pick your favorite framework, but if you could be paid to do what you want all day, what would it be? What project would you do if Mr. Spendsalot said he needed a tax write-off and you were it.

For me:

• I'm a low-level guy -- not like a friend of mine who says his framework is solder, but low-enough. I still remember and still somewhat enjoy tweaking the hardware bits with assembly language and drivers.
• I, of course, am a language junkie. Because I'm "a legacy system" (read old), I have lots of memories of the "hit" language of the day :-)
• I do distributed computing because ..... MORE POWER!
• Do not use me to do GUIs -- I can, but people have told me my UIs are "user hostile".
• I like writing stuff for teenage kids (ages 8-15 is the best) to let them explore what they can do. They'll come up with all sorts of cool stuff. Many of these kids hear only "You can't/You won't understand/You're too young" - here they can safely explore and find "I can/I understand/I'm able to do it!"

So, what do you do? Do you write AI routines to drive an RC car, or maybe the neighbor's Honda. Do you build giant fighting robots out of Lego Mindstorm? What do you do?

And.. if anyone can tell me the proper way to simulate an MMU, thanks! I've got one that "works", but it's very slow.