When at all possible, let others do the work. Yeah, that may sound lazy, but I prefer to think of it as efficient. Truth be told, there is a LOT of quality educational material out there that is free to use. A lot of it is for more advanced users (college courses and the like), but some of it is perfect for any age.
Here are a few of my favorite Open Educational Material sites
This initiative from Rice University provides dozens of best-of-breed college courses free for the public.
From MIT, another exceptional site. This one includes actual MIT courses that are streamed. You can interact with other students, ask the professor questions, and even do the assignments
Udemy is a commercial site where users upload courses on every topic imaginable. There are a lot of these kinds of sites, but I chose this one to post about because it has high quality standards and a wide variety of courses
The Holy Grail of free online courses from top-of-the-line Universities. Some are always open, others run at the same time as the class on campus. Interact with students and professors and even take a credited route.
What have I missed? Share your favorites in the comments.