Well, most of my suggestions are out since I do some very "obviously not work related" hobbies at work (think of my job like being a fireman... I have certain duties that have to be performed, but my job is really about the emergencies, and I get paid for the knowledge and ability to perform during a crisis, not so much for what I do with every minute of my time. One part of my job involves sitting in a shack and pushing a button when it is time to do so. It's involved enough to require training and what to do if things go wrong, but physically it's just pushing that button and making an entrance into a computer. Each time I have to go out to the shack to push the buttons takes about an hour from start to finish, with less than 5 minutes of actual work. I cannot leave the shack. So I knit, crochet, hand quilt, read, watch videos, work on spreadsheets, etc.)
Anyway, just be careful about doing anything as a "side hustle" while on the clock. There may be a rule forbidding that, and doing so could cost you your job. I can craft my heart out, but I cannot turn around and sell said crafts.
If I were stuck in an office job with that much downtime, I'd see if working from home a few days a week was an option...
ETA: I'm in a position where I have no intention of getting promoted, being a star, and motivation has pretty much been beaten out of me, repeatedly. I do my job, and do it well, but don't go above and beyond anymore. If your company rewards self-starters and values initiative, don't be like me. My job is great not because of the job itself, but the amazing schedule and downtime, once you get over the fact that you have to push down your natural drive to strive to be awesome.
