First of all, see what prizes you have, how many compaired to how many people you have. Prizes should be one of the last things you should worry about. Get the date of the event, a place to hold it, and games in slots before worrying about prizes.
Prizes I did two ways. The first Gameday I gave a prize to all the DMs and then had them give out two books based on whatever they wanted. Some of the people didn't like that idea.
The second gameday I actually had more prizes then people so everyone walked out with at least one book and many people got two. I had them roll the magical d30 when they arrived to determine what prize they got.