To offer an alternative view: nothing! It sounds like you had a great time with the game, and it’s sometimes nice to end on a high note instead of after trudging through a bunch of side content. Besides, I’m sure you have some other games you’ve been meaning to get around to 🤔

I’m not sure what interests you most (or if you’ve already done this), but one thing I’ve always enjoyed is trying to tackle all of the shrines. Each of them has their own puzzles that’s different enough to keep me entertained, and the access to skip travel points is great for whatever else you might be doing.

You can also try to get korok seeds but those are sometimes even more annoying than the regular side quests

Edit: Sorry for the double comment, my app was glitching

Go find all of the armor pieces in the game. Some are quite tricky to find. Then if you want the extra challenge, upgrade all of that armor to its max.

I got all the shrines and all light roots. Then I started messing around with building combat vehicles to mess with bokoblins. I'm not a big side quester or story liner, so I haven't done a lot of them.

I got bored at some point since the objectives were completed. It's sad, but a game is over when it's over. While it's possible to continue through speed running or going real 100%, it's fine to just be done and move on to another game.

I completed all of the shrines before I beat the game, and found it enjoyable. I also really enjoyed running around the depths collecting all the lightroots. I enjoy exploring caves and wells too, so that’s next on my list to complete. Grinding for armor sets is tedious to me so I’m skipping it…

Go get Majora's Mask. Hardest fight in the game.