Welding - get a decent MIG welder and you can probably learn enough by watching youtube to stick metal together fairly well. If not, find a local welding shop and talk to them - most of them are more than happy to pass on knowledge, and some may offer some private lessons after work for a fee of some sort. My neighbor owns a shop and he wants to start doing this at some point because he both loves welding and feels a need to pass on knowledge to the younger generation.
Machining - I bought a metal lathe a while back and love to play on it when I can. It's not cheap, and takes a lot of patience and knowing yo uare going to eff something up while learning, but it's something I love to do. Hell, learning any type of skilled trade really is for me, even if I know I'm never going to be more than decent at it.
Reloading, bullet casting, etc. Great hobby, relaxing and an excuse to test out your work.
Blacksmithing - what my next venture will likely be. Working with metal is fun, it's an excuse to buy new tools and learn something I deem useful. You'd be surprised at how many things you find to do or make or fix when you have desire and tools to do it.
Woodworking - agrams's posts just make me jealous. I'm probably going to have him make me a couple of simple things as well. I'd kill to have his setup and skills. I could eventually get the setup part done, but I'll never have the skills. Still, there's something satisfying about being able to make or fix something yourself.
Gardening - would love to have the time, but i likely never will. The idea of being able to eliminate a lot of my grocery store expenses is appealing to me.
Pretty much you need to find something that piques and holds your interest that you can enjoy doing. Doesn't really have to be useful or anything of that nature - but it needs to be something you will never view as "work" and will always view as "fun". That's the key. Doesn't have to be one thing either, can be a lot of things.
