The idea is to take an Open Source Textbook on mathematics, physics, chemistry or biology and make an Linux style inspired podcast series. Here two or more people discuss the theory on each of the topics, problem solving techniques, its applications and more.
Even if this is a podcast you can have visual content. Just add links to images/videos referenced in the podcast to your web page. It should make it less expensive then to do a pure video podcast.
I think following a podcast for your textbook where they ask interesting questions, where you can learn from good explanations or maybe just get motivation from a fun session could go a long way.