Everything introduced in US/UM:
Mantine surf is new, along with the surfing beaches
The Battle Agency
The replication of Lt. Surge's Gym
Totem Stickers, and the subsequent gifts that go along with them
Necrozma is focused on more, including more story and Ultra Necrozma.
Team Rainbow Rocket
Poipole, Nagandel, Stakataka, Blachephalon, Ultra Megalopolis, and everything related to Dulse, Zossie, Phyco, and Soliera
General tweaking of encounters, trainers, and items
Hope I helped! I would personally recommend Ultra Moon over S/M.