I always tell people to investigate drywall taping. Not the most glamorous job, but it pays well, only needs ~$250 in tools, it is not hard to pick up cash jobs on weekends and it is easily portable if you move. There is an apprenticeship, but you will get crapped on a lot less than if you moonlight in plumbing or electrical without the proper certification.
Wouldnt want to be doingvthat when Im 50 +