I'm mostly playing two games right now: World of Warcraft and Reader Rabbit Preschool.
Reader Rabbit is very challenging. It's not that the tasks are hard, it's just that the interface is really complicated. You have to install a child in front of the keyboard, physically point at the numbers and letters and shapes on the screen, and verbally tell the program which answer is right and why. Once you build up enough experience points, some kind of learning subroutines eventually kick in, but it's a very slow process and there isn't enough precise numeric feedback. Requires more patience than most gamers have.