Hi Cindy - with that large of a party you are going to have your hands REALLY full. In this case, less is more especially if your backyard isnt a half acre. : ) You might need to enlist your husband or another parent to help you coordinate the games/fun while you welcome guests and deal with food and the birthday cake, etc.
If you dont want the pinata, then dont do it. I dont think 3yr olds have the most fun doing it. The kids are going to want to spend most of their time in the bouncy house anyway. I like the hidden candy/treasure idea.
How about a craft that the kids can work on with their parents like a foam craft or something that fits the theme of your party. There was one party we had where the kids decorated foam visors with foam stickers and letters, etc. The girls could turn their's upside-down and make them into tiaras. It was something that any age could have fun with.
If you want to do an organized game, just consider that you will have 12 kids so they will have to wait a while for their turn.
Have fun!