I took 2 of my kids (12 and 8) to Universal and Islands of Adventure for the first time a few days before Christmas. We had 1 day to explore both parks and we did not get express pass (didn't stay at one of the resorts that offers it and it cost $150 per person the day we went, which is way out of our budget.) We had a wonderful time and I felt like we got a lot accomplished, even with Christmas crowds. Parks were open 8-10pm that day and we were there the entire time. Headed straight for Gringotts at arrival (we loved this ride.) After Gringotts we did all of the following:
Ollivander's wand shop show, Hogwarts Express (in all we rode this 3 times throughout the day- amazing), tried ButterBeer, all the rides in Suess Landing, saw Grinchmas show, son rode Dragon Challenge, had quick lunch, we all rode Flight of Hippograff, watched the baby Dino hatching in Discovery Center, did all 3 water rides as it was quite warm on this day (and got drenched/luckily brought extra clothes), rode Mummy, Rip Ride Rockit, break for big pink donut in Simpson area, Men in Black, ET, the little Woody Woodpecker coaster, watched some of the Macy's parade/floats, Despicable Me Minion Mayhem, Florean Fortescue's ice cream break/more pics in Diagon Alley, Spider-Man and finally son did Dr. Doom as park was closing. Line for Forbidden Journey/Simpsons was always very long and to be honest, I was hesitant to try these rides without my husband in the park in case they would make me sick (I still had to drive kids and I back to resort.) Then we had a late dinner in CityWalk. I would think with the Express Pass, even with crowds, you should be fine riding the headliners.
Have a great trip!!!