Lots of great recommendations already in the thread. DH and I enjoyed a couple of drinks/snacks, just sitting between the NY and SF areas. Love that the themed areas are still there, stoops to sit on, all the shops, etc. We LOVE GE at Disney's HS, but miss the old theming of the NY facades of MGM. Universal still has so, so many areas still themed like that. Take the time to take lots of pictures!
Last couple trips, took time to try the unique brews in HP, Hogshead brew from the Hogs head in IoA, and Dragon Scale from Hopping Pot in Diagon.
eat at restaurants the kids don't want to go to. or just snack from festival booths if there's a festival
Take all the time you want to go through the queues, and roam around. Molly from Allears has such great videos on HP easter eggs, etc
watch all the shows, Blues Brothers, etc. and the HP ones. Do the meet/greets, etc.
would love to try one of the behind the scenes tours. Sounds fun!
DH doesn't like to do this, even at Disney, but I've enjoyed going around to the various resorts and just looking around, maybe get a snack, but only if you have extra time. wouldn't take park time away for it.
Have a great trip!!!!