One of our family's favorite traditions is the half birthday cake. When one of us has a half birthday, we bake a cake and sing "happy half birthday to you". That's it. Nothing else special; no presents, no games, no nuthin'. But the kids love it and then they know when they can start telling people they are their age AND A HALF. Plus, the way things work out now, it means we get to bake a cake either for a birthday or half birthday every month except January and July. So if we ever have another child (which isn't likely) we'll have to aim for one of those months.
Here's Maren with her half birthday cake. And yes, it did eventually fall over, but it was yummy!