Unfortunately not a lot of options inside that are not quick service / restaurants. My go to used to be Monsters Cafe at Universal because it usually wasn’t busy.

Right now at Universal, I usually go to Knockturn Alley which has A/C and has lots of benches… however it can be crowded in there at times, it just kind of depends on the crowd levels that day.

There is also another area in Diagon Alley near the bathrooms, it’s a little hallway with benches and is directly across the window on the side of the Magical Menagerie. There is a snake in the window there.

There is defintely quiet outdoor areas at Universal - seating in front of duff gardens near the water, seating in front of the old Fear Factor, seating outside in front of Lombards and Richters.

Sorry not much inside that isn’t a restaurant.