Honestly? Pretty much everything. And thus in effect I get nothing done in my current level of free time as I'm jumping back and forth between dozens of fun projects.
I'm aware this may not be a satisfying answer, but it's the truth, for better or for worse.
It's satisfying for me, it's an honest answer, and as good as any other.
With enough free time there wouldn't be much need for a direction. And after a while I would probably figure out something I liked doing more than other things.
Only issue is that in my eyes, enough is probably pretty close to immortality.
Largely because I would like to understand the breathd of the human condition, and how language makes us think differently. That probably requires getting to native speaking ability in maybe 100’s of languages, and also the patience to wait for the likely needed cognitive augmentation to be able to do that.
I would also very much like to visit a few stars and nebulae, which could take a while.
If I only had a couple of decades or so, I might take a stab at building tools to make programming the job it should be, but isn't now.
I'm aware this may not be a satisfying answer, but it's the truth, for better or for worse.