I’d draw various inventions and work on sketching skills; writing is too much work!
Optimizing paper airplane design could burn a lot of time too.
I’ve been meaning to write a “to do” list for decades. Maybe I’d finally get around to it.
Mathematical research. I would try to find something to work on that I didn’t depend on the literature.
Reminds me of school. I wrote lots of short stories and developed my own little sci-fi universe while pretending to take notes during classes that were painfully boring.
I always felt like I wanted it to be a novel if I had time, so that would probably be it.
I would write - or attempt to write - poetry. It’s something I used to do when I had such a thing as free time and an uncluttered mind. I would imagine that solitary confinement would force both of those on me, so I might as well put them to good use.
I’d make flipbooks. Try and recreate all the SF classics I can remember in flipbook format. Maybe write some of my own.
Solve the Riemann Hypothesis.