Animal husbandry - goats. This is something I probably won't invest in until next year when I have some time. But the idea is to raise them free range (grass-fed) and harvest the meat in a way that is truly "green" and "organic." Then market and sell the products (meat, milk, cheese, yogurt, etc) as organic at gourmet farmers markets.
Commercial mushroom harvesting - there are more than 10 species of commercially harvested mushrooms where I live in the Pacific Northwest. Learning to find and identify the valuable ones and when to harvest. I have been going out and doing this some on the weekends this year.
I could go on and make a grocery list of subjects, but I'll stop there.