I grew up "sort of Christian" as well and I still celebrate Christmas but under no religious basis. If you do have kids, then you can just treat it like Easter as I'm sure most secular families do. Easter is about the Easter bunny and finding eggs and nothing more. Most people don't even know why we celebrate Easter. So for Christmas, you can just have the whole Santa and Rudolph thing without Jesus and your children will probably enjoy it as much as you did when you were young. Kids aren't excited about Jesus when they're young, they are excited about Santa and Rudolph.

btw, Festivus is not a bad idea. If I were ever invited to a Festivus celebration I would surely go.