Just the basics! Being a Master Gardener, I have helped to educate people about gardening, pruning, etc., and it seems the most widely appreciated information is very basic. Not all gardeners are as "die-hard" as you or I may be and they love learning the simple things. This helps them to properly progress into more advanced plant care. I can't believe how excited some people become just learning the benifits of mulching or when they take my advice and fertilize. Their plants start to really grow and thrive and they can't believe that's all it took. Try teaching a class that revolves around gardening basics and i'm sure it will fill up fast and you will get lots of thanks!