I solved downtime, and the mental boredom it created, by getting a new job.

For me this happened when I lost a sense of understanding and purpose for what the job was. Previously, I understood the larger goal of the team, I had autonomy, and when there weren't urgent matters from management, I would have no problem writing docs, fixing bugs, or coming up with new ideas to move the needle in the right direction. Knowing the larger vision and being able to see the impact of my work made this all feel effortless.

Fast forward to now, and I don't understand the vision of where we're going (I don't think management does either), any time I try to do something without the blessing of management I seemingly guess wrong (sometimes I think they make it different just so they can say it was their idea), and every project feels like it exists to check an arbitrary box. This all leaves me feeling much like you. I've come to the realization that no amount of effort on my end is going to change these issues, short of leading a coup (which I'm not going to do). This leaves me with the option of dealing with it or finding a new job.

If you have clarity on the vision and some autonomy, I always found it useful to focus on the customer. Who is going to be using this stuff you make and how can I can make their life better/easier. Sometimes that is documentation, sometimes that is fixing annoying bugs, or refactoring to make things faster. The refactoring can also be to make my own life better. I also found it helpful to have a little side project at work. When the main job started to feel like a grind or something I just didn't want to do, I could hop over to my pet project for some quick wins I enjoyed, and carry that momentum forward.