I finished the postgame in sun, and I'm really bored now. What do I do?
Things I've finished:
Thanks for the help!
I'm assuming since your pokedex is completed you already got Type Null from Gladion and caught the UBs and Necrozma. If you like hunting for shinies, since your Pokedex is complete you can go to the Game Freak Office on Akala (It's in the building next to Dimensional Research Lab) if you talk to the Game Director he'll give you the shiny charm which makes it easier to find shiny Pokemon. You could breed competitive Pokemon and battle online. Other than that my mind is pretty blank. US/UM will probably have a lot more post game so you should look forward to that. I hope I helped!
Along with what ShinyZebstrika113 said, you could also go to the Battle Tree and win Battle Points. Then you can exchange them for useful items. You could also try training new Pokemon until they are strong. What I like to do sometimes is go to MeleMele island and pretend I'm starting the game over again with new Pokemon.