Fun depends a bit on their aptitude and interest. DS8 enjoyed BrainPop for about six months, still uses Art of Problem Solving's free Alcumus for intersting math problems, the school has quite a few different subscriptions that he explores on and off. Wikipedia fills gaps here and there on all sorts of topics. He also had fun with a Lumosity subscription. Howstuffworks.com has some great material. If you are looking more for hands on, instructables.com has a ton of user generated material of varying values. If you have Amazon prime or Netflix, they both have a wide range of videos. Some of these obviously are quite open ended and require some self-driven curiosity to get the most out of.
