brain teasers

Khan academy is an amazing resource.  It's a HUGE collection of videos, all featuring a smart man with a gift for clearly explaining the complex, carefully going through a wide range of topics, all clearly organized.  There's everything from classic brain teasers, to arithmetic, to high school level math, including calculus and differential equations, to biology chemistry, physics, and economics.  The graphics in the videos are fairly simple, and the tone is very friendly, conversational and often funny.