Reading is always good, and waay better than scrolling. Audiobooks/podcasts are also cool and if you can't fully commit yourself to writing maybe just jot down an outline to flesh out later

Sometimes I play sudoku rather than doomscrolling.

Gutenberg.org and go to browse / random Do mundane things like make shopping lists... Look up recipes to cook for the weekend. Get a note taking app and organize lists of things you need to get to or want to get to. I use Google keep but there are a bunch. Research things you always wanted to.

I like podcasts at work

yeah, eBooks. I keep a few on my phone and just work my way through them during downtime. I use the ReadEra app on Android, and mostly read copyright free classics at the moment.

I was going to suggest reading but you already said it yourself. So just go with that. I think it's one of the best uses of free time a human has.

Chess on your phone. That's what I do. Podcasts or audio books while on the move

How many times a day are you capable of beating off?

Practice juggling. Buy 3 juggling balls (you know those bean bag types that don’t roll away), and keep them with you. Don’t use tennis balls, because they have a tendency to bounce and roll. When you have some downtime, start practicing. Watch a few video tutorials to get the hang of it.

If you can have a backpack with you, consider bringing a tablet with you. If you have one with a pen, you can draw. Watch a few more tutorials, and learn to draw something you like. Landscapes, portraits, fantasy dragons, pokemons whatever.

Tablets are also fine for reading ebooks. Check what your local library has to offer. Maybe you can borrow library books and read them on your tablet.

buy an emulator (miyoo mini plus, if I could suggest one) and have fun!

I once had a data entry job where I got through a couple of audiobooks a week.

I listen to educational YouTube videos during work. We don't have down time but we have no supervisors on my shift. I can get my work done while listening to videos.

Get some audio books / tts books / podcasts and craft away. You could even start etsy.
Easy to put away and clean crafts: crochet (amigurumi), knitting, needle felting, tatting, embroidery (cross stitch), hand sewing (English paper pieceing), sketching... There has to be more that I can't recall... Macrame needs too much space.

so you can study stuff and do coursework but you gotta make it really easy to do accidentally. when I got a paper to write I would make a google drive folder with the project outline and a template for the paper including a reference sheet. I would lazily look up sources occasionally and add the pdfs to the folder and easybib / citation machine them into the reference section by doi. at that point it would be a matter of when the paper would get written, not if. the dopamine would just hit suddenly when I'm having an IBS shit at 2am and "oops. I wrote a research paper." I accidentally wrote so many papers while shitting. one time I set all of it up and went to bed and turned out one of the group members was also ADHD because when I woke up she'd accidentally done the whole thing while I was asleep. 10/10 method.

Buy a Steam Deck and get into Skyrim or something?

I read an ebook. Mostly fantasy but I am reading some cosmic horror at the moment.

Chess puzzles

I do read, go for a Pokemon go walk, browse on my phone or listen to music when I get a break at work.

You could look up paper airplane designs and try making some.

I can’t recommend getting an e-reader more. It’s so much nicer than staring at a backlit screen for extended periods of time.

Stare.

Or the ol classic: If you got time to lean, you got time to clean.