I undertook some formal training in my up-till-then hobby, a couple of years before I retired, and also gained some training qualifications. Then, on my last day at work, I was offered, out of the blue, a short-term contract for some interesting work.
The most satisfying things, for me, have been training newcomers to my profession - I was approached by a college to do one day's work a week - and working almost full time at my former hobby and becoming reasonably proficient. Mind you, I'd hate to have to rely on either for my living, but they provide reasonable pocket money.
Training young adults, I found, is great. Mostly, they are more motivated than school students, you don't have to treat them like children and they are often doing interesting things at work, which helps keep you in touch with what's happening. When you see them succeed, you can say to yourself "I helped him/her do that."