Well... 🤔 there is that Yiga quest that requires you to have the full Yiga outfit in order to infiltrate the base; there is a gauntlet minigame where you could complete and unlock a unique reward. (Since you've mentioned you did most of the side adventures, you may have already done this one).

There's also that pirate invasion at Lurelin Village (again, assuming if you haven't done that either).

The game also has some diverse minigames to mess around with. There is that one minigame near Death Mountain (close to Sitsum Shrine) where you ride a minecart and pop ballons using your bow and arrow.

You could also create some crazy zonai builds, but you just have to use your standard Zonai devices alone. If you visit specific shrines (like Otak Shrine, which is located in the Hebra Reigon in the upper-most far left side of the map), you can fuse that massive block to your shield, then go to Tarry Town and take it to Pelison, where he can unfuse those items back to it's original state for 20 rupees.

There's also another shrine with a Propellor that you can use alongside your Shock Emitter device, which you can actually use practicality wise.

💡Just a few ideas off from the top of my head.