What are good ways to fill downtime at work?

Depends on the environment at work (do you have privacy) and your interests.

• ebooks on your phone or a reader (or even a physical paperback!)
• games on your phone, or a small console like the retroid pocket 5
• learn something (new language, watch a lecture course, etc)
• learn + practice some IT skill (programming, website design, etc)
• learn + practice any other acceptable skill (drawing, writing, etc)
• watch movies/TV shows
• get home tasks done to save time at home (home admin, pay bills online, etc).
• chat with someone you know

Self process improvement of work task. If they lead to new knowledge better.

Coffe with coworkers.

Do stuff to make the moments of heavy load feel lighter . Like organizing or advancing tasks .

If you've got time to lean, you've got time to clean

If you work in an office, you could make coffee for your coworkers or clean the coffee pot.

Read a good book, chat with coworkers, build solidarity and unionize✨

Good: Resting your eyes, listening to a book, tidying the workplace.

Better: Anything creative, phoning a friend, writing or drawing in a notepad.

Best: Stretching, going outside, drinking water, going for a stroll.

Extended toilet break, with your phone

I try to freelance in my downtime as much as possible without getting caught 😂

Complain about how slow it is / try to convince coworkers to go home so I can capture a greater percentage of sales.

Depends on whether at office or WFH. In the former - scroll a book/Lemmy/Wikipedia or text because there isn't much else you can do. In the latter you can do anything, I do my hobbies, grocery shopping, chores and basically everything else during work so I can actually chill after 5pm.

Prank your coworkers. Buy one of those noise makers and hide it somewhere.

For me it's usually chess, or shelled into my homelab to tinker with one of my personal projects