I probably won't even tell them about Santa unless someone else does, and then I'll tell them it's just something silly and fun people do during the holidays but he's not real.
for Christmas, we'll bake cookies on Christmas eve. We'll get our tree a week or two before Christmas and decorate it together. We'll do Christmassy crafts and decorate the house with them. We'll have lots of food and other fun things on the actual day of Christmas.
I haven't thought about other holidays yet, but I'm sure once I have kids I'll think of thousands of fun little things to do.