I've always used my calendar for some boring scheduled tasks - I moved those reminders to 3pm on Fridays. I also made a list of things to do when I hit that 3 or 4pm "ugh I feel like doing nothing" mental state, which definitely happens. My list is:
1) Read all memos that we've gotten from suppliers lately. Since they get saved in special folders by people I have a habit of being like well they're saved forever, I can read them any time in forever... but that didn't always happen.
2) Check inbox for things waiting on my signature
3) Clean desk
4) Clean computer desktop, "my documents", and download folders where files just seem to seep
5) Revisit top priorities list
6) Spot-check personnel files to make sure folks are documenting recent accomplishments
7) Erase whiteboards. I hate when the marks get left there for months and don't erase. Plus people should be using them to think, and clean whiteboards encourage thinking - especially in those fighting meetings where everyone is stating and re-stating their opinions, I learned this from an IT guy, the second a fight starts go to the whiteboard and start writing down what people are saying. When they see their opinion up there, they drop the need to re-state it.
8) Catch up on industry and competitor news
9) Set up 1x1 meetings with random people - this was a goal from 2014 or something, if you're constantly calling people into your office to talk about improvement, they don't get scared on days when you HAVE to say "Come see me in my office".
DONE. That's enough things to keep me plenty busy and moving even if my brain is tired.