We've had breakfast and lunch at The Three Broomsticks. Well worth the money, just for the experience, plus we enjoyed the food. Frozen butterbeer is better than liquid, but you my want to try both anyway. Cauldron cakes at Honeydukes are delicious. The all day dining is a good deal if you are going to eat a few times (more than 2 and i dont think any of the restaurants that except it serve breakfast) and you don't mind the fast food type food that comes with it. There is a bakery on the road just inside the gates called The Croissant Moon; Great pastries, breakfast sandwiches, sandwiches and desserts. We eat there a lot. Lighter fare than the burgers and chicken strips at most places.
Don't know if you plan to get the express pass, but you can't use it on Forbidden Journey or at Ollivander's. You can use it at Dragon Challenge, but the regular line has all kinds of interesting Potter things to look at. The express line does not. You can't take anything on Dragon Challenge, not cameras not drinks, but when we went the guy checking express passes told me to walk thru the regular line to get more pictures and then I just walked back out. Forbidden Journey is a walk thru as well. If your camera is small there is a little compartment in your seat that you can store it in, but nothing else can go on the ride. Both rides have free lockers you can use to store your backpack, etc. The Flight of the Hippogriff is a kiddie ride, but you get a great view of The Wizarding World. You can use the express pass here, too.
There are several stores with awesome stuff. Ollivander's for the wand choosing is fabulous. Owl post, where you can buy stationary and have letters or postcards postmarked from Hogsmeade. Honeydukes and Zonkos.
There will be crowds, but if you are patient you can see it all and more of the park besides.
If it were me and I only had one day there, I would focus on the Wizarding World.
I would:
Eat at least 1 meal at The Three Broomsticks (preferably 2)
Get a drink at The Hog's Head in the souvenir mug
Get frozen butterbeer in the souvenir mug
Get a small glass of regular butterbeer, maybe with my meal
Get pumpkin juice
Get cauldron cakes
Get chocolate frogs
Go on the Forbidden Journey ride
Do the walk thru of Forbidden Journey and see how it changes
Do both sides of Dragon Challenge
Ride Flight of the Hippogriff
Go see the wand choosing at Ollivanders
Send a postcard via owl post(bring stamps. They only had souvenir stamps when we were there)
Check out the shops and buy loads of souvenirs( they have racing brooms for sale. $375 to ship to Canada)
Watch the durmstrang and beauxbatons dancers.
Take lots of pics