We're there Dec 25-28, first time to see Diagon Alley. FWIW, here's our plan:
Arrive late morning on Christmas Day, check in (don't expect room to be ready), pick up our passes, head to DA to "see the sights." DS and myself won't be able to wait to see DA

so we'll focus on soaking up atmosphere that first visit. And get a butterbeer. Take a park break midafternoon to move into the room and have an early dinner, then head back into whichever park we want.
Boxing Day, we'll rope drop US, do Gringott's, then Hogsmeade Express to Forbidden Journey. From there, we'll wing it. (This is one of the things I love about the Dark Side. Even at one of the busiest times of year, I don't have to pre-plan everything!)
On past trips (including Easter week), if we weren't at the front of the pack for rope drop, we found one of the best times to do FJ was right at or just before regular park opening. Most of the RD crowd was through the ride (or the line was shorter than it had been 30 minutes earlier), but you still beat the regular entrance folks. I'm hoping for a similar experience this trip, and also for Gringott's.