Ah, my son is a mere three and a half, and this is going to be his second Christmas that he kind of understands.
I introduced Santa last year under a sort of duress, and it's kind of humorous. I had a collection of dried poppy seed pods my son was systematically asking to play with, and then destroying. Because they were meant to be a decoration, I decided that while I still had a few left, that he shouldn't have any more from me, but could 'ask Santa' for one.
So we wrote a letter to Santa, made a big deal out of mailing it from the big mailbox down the street, then celebrated it by going to the local pizza place. Husband and I agree that this is a great idea for a tradition.
Sure enough, Christmas morning, Santa had left a seedpod in his stocking. It was promptly destroyed, but it wasn't mine, so we didn't ask for more. :)
My husband and I keep Christmas simple: stockings for each other are usually chocolates and a bottle of favorite import beer. Kiddo got the seedpod and some fortune cookies, which are a big deal for him. Then we had the rest of the gifts. We do people gifts under the tree in the days before, and Santa does stockings. Even the cat gets one!