- You can capture Solgaleo
- You can capture the four Tapus, as well as get Tapunium-Z
- Take down Team Rainbow Rocket at the Festival Plaza and Aether Foundation.
- Post game area at the last island- Many new trainers and Pokemon.
- Battle Red and Blue near the Battle Tree
- Unlock the IV checker near the Battle tree
- Battle for points at the Battle tree
- Meet the trial captains, and do what they ask you to. You get stuff like the starters Z- and other things.
You can capture all the UBs, Necrozma and Legendaries (Box art and side Legendaries of each game) in the Ultra Wormhole, but it isn't post-game content. You can do it at anytime.