I believe postgame we have a lot for pokemon Sun and Moon.
We have:
. UB sidequest
. Battle Tree
. Defending your title of Pokemon League Champion
. Rematches
. Battle Royale
.Changing the Universe to Moon Universe and vice versa
We have a lot and plus we get rematches on a daily basis with a lot of people you normally don't get in past games Like rivals (Gladion,Hau) Professor, Team Skull(Guzma,Plumeria), Kahunas, Elite Four, Past Characters(Red,Blue and other characters). In the end I believe these are the best games of the series because they have a really good mainstory and postgame plus it fills my headcanon that it tells us what happens with some characters and what they decide to do with their lives postgame. Like Hau looking for his father, Lillie going to Kanto, Gladion becoming President of Aether Foundation and the Protagonist not only becoming the Champion but being a active Champion as well!