It's not complete Tasks, but I find if I want to give my brain an exercise to improve my programming ability, I just hop onto StackOverflow and start answering questions in the tags I'm interested in
In addition to learning stuff, I also get to see what other people's solutions are to the same problem, what other people think of my solution (votes), and I get to help someone out in the process :)
I also like the fact it's short-term and non-committed. If the problem is out of my depth or if something in life comes up suddenly, nobody cares if I don't actually come up with a solution.