OH WOW... the parks are going to be so crowded!!! The celebrations will be huge... Why??? Its my birthday hehehe.
In all seriousness we go every year over my birthday and we don't find it too awfully crowded. Leave an hour and a half to an hour and 45 min before park opening if your heart is set on getting there right when it opens (this also depends on how well you know the roads). As for things you can't miss, lol, you can't miss anything!!! I suggest you go through the Theme Park portion of this site, as well as
www.wdwig.com, see if there's anything you cannot stand missing. I would say not to miss any of the major attractions in any of the parks of course.
As for things you might not think to take I suggest looking through the packing lists here
http://www.wdwig.com/forgot.htm both for what you would take to the parks and what you would pack for the trip. My family has gone to WDW since before I can remember, but I remember my wife telling me about going with her drill team in high school, none of them had been before and they didn't study up on what to do. They had a Blast!!! they just ran around the parks doing everything they saw. When I took her for for 2 weeks as part of our honey moon she was amazed at what more there was to do that they hadn't seen. What is my point??? Just go and do anything you see, anything that looks fun. When you get the chance to go back you'll be a little more prepared to search out all the things you didn't get to do the first time. I hope you two have a great trip!!!
PS.... I thought of something not to miss. Ice station Cool, its a coke display where you get to test coke sponsored drinks from all over the world, this is a great place to get a free drink!!! I'll just warn you about a drink called Beverly, thats all I will say wont say its bad or that its sooooo good that you will be addicted.
Anyways this is located in Epcot, as you enter epcot you will walk past spaceship earth (the epcot ball) next you will see a huge water fountain go on the right side of the fountain and you will see an Ice cave just go through it and it will take you to the store!!! again enjoy