Online courses offered free by various universities around the world. Irresistible. 4 to 6 hours a week? Easy. A far better use of spare time than playing games, that's for sure.
Straight away I signed up for Social Psychology. Then I noticed that Genetics and Evolution had already started, but only just over a week ago - I can catch up.
EDIT: I caved and signed up for The Modern World: Global History since 1760 and very glad that I did, as it is absolutely awesome, but I am looking at others:
SECOND EDIT: This history course is perfect. It really is. If you have any interest in history, or why things are the way they are, I promise you'll love it. It costs nothing, and the material is so easy to follow. I cannot recommend it highly enough.