Time. That's what you've got a lot of in the FA zone.
I can sit doing absolutely nothing, just staring around the room, noticing a cobweb, listening to the street, just letting my mind wander. I'm old enough now that I sometimes fall asleep for a nap. I never used to be able to fall asleep in the day. Naps are awesome.
I really enjoy just pissing about on the computer. I can spend a happy hour reorganising my files on my computer, and then do it all again a different way the next day.
I enjoy the Civilization PC game series that has regular updates and mods across the last three versions (III, IV, and V). There's always something new to see and do.
I've decided that the age of 45 is the perfect age to get into comic books for the first time in my life. Comic books, graphic novels etc. weren't as much of a thing twenty-odd years ago when I was in the core demographic. They look amazing today.
I watch TV and films. I can get through a season of a TV show in a few days. I've got more movies lined up to watch than I'll ever get through. I go through periods of not being able to watch TV shows and films, because they mostly feature 'characters' who all have 'lives', and if I'm in a glum sort of mood they make me feel worse.
I do have more grandiose projects that I work on every day, but never seem to go anywhere. Writing is a big one and I doubt that that's ever going anywhere. But so what? The double-edged sword of FA makes you your own judge. Nobody praises you, but nobody blames you either.
You will need a sense of purpose to survive mentally intact if you stay in the FA zone beyond your 20s.
Cultivating hobbies and interests is recommended all the time here. It's good advice, but I have one reservation. Don't think of them as 'hobbies'. You'll likely have little or nothing else in your life apart from building your matchstick cathedrals, or your obsessive reading of everything by Charles Dickens, or whatever it is. I think you have to take the pursuit and enjoyment of 'hobbies' as seriously as you would take a day of hunting if that was the only way you were ever going to get food. The stakes are just as high.