I'm slowly (very slowly) working my way through the astronomy course (96 lectures!) as well as the visual tour of ancient Rome. No way to do those on audio, I'm afraid. The art history courses are also excellent.
I’d like to have some of the archaeology and art series on DVD. Maybe Geology, too. My 12-year-old is nagging for the Peloponnesian War set; I could get it for him at the end of the school year, if he’s done well!