Hi how exciting for your first trip! I can't wait to go back every time I get home. Everything (LOL)! There are so many little details in everything in those two parts. They have captured the essence of those two places. You feel like you are right there.
Olivanders is great to do in either part. The interactive wands are great to use.
Use the bathrooms in Hogsmeade and pay attention to any sounds or voices you hear.
Florean Fortescue's Ice Cream Parlour in Diagon Alley (awesome ice cream). We enjoy eating at both restaurants.
Here's a tip for Hogsmeade if you are going to get butter beer, frozen butter beer or pumpkin juice on tap (really good) go over to Hogs Head tavern inside. There is usually not a line. The carts out front get very busy.
Check out all the stores in both worlds. My DD and I did this on the last trip and we noticed there is some merchandise in some that are not in others. We do all our looking then go back and buy.
Take the Hogwarts Express both ways it is different.
Make sure you check out Grimmauld Place outside of Diagon Alley. Look up in the windows.
Since HP worlds are your top priority I would get there as early as possible when there is less people. It is so much nicer to really see everything. Of course go back in the early to later evening it will be less crowded and beautifully lit up.
If you go back in this thread if you haven't already read the HP Wizarding World FAQ thread.
You will be amazed when you are there. My DD and I get goosebumps when we walk through the worlds, especially Diagon Alley. Enjoy