Skills learned, particularly manual skills, though mental ones pay off, too. Fixing cars (and tractors), building things, growing and cooking food, sewing, managing the flow of water in a landscape, appliance repair, plumbing, wiring, etc.
The latest on my list to get good at: chopping wood. It was brought home to me recently that I lack this skill and need it.
Calculating ROI is hard for the ones that cost only time, but the value is immense.