Christmas Eve - we only do new pyjamas but have a tradition of eating certain food for dinner.
Elf - I got loads of ideas off Pinterest so was completely prepared. He mainly hid and they had to find him, Dh was also great at moving him around. I bought the traditional one and performed surgery on him so he would have posable arms and legs. This showed me that he is cheaply made with a roll of crappy cardboard and a bit of stuffing. I was worried he would die on the operating table.
He arrived bringing cereal we would never buy
and was limited to downstairs as he would freak the children out otherwise. The message was truly that you can be a really great kid but sometimes you can do naughty things, but it doesn't make you naughty. Father Christmas welcomes back the Elf.
A word of warning, don't think that the minute they stop believing in FC means the Elf stops. I finally stopped it last year when my sons were both teenagers! Mainly because he felt like tradition. So be aware that what you start, you may never stop. They still hang stockings for Father Christmas on the fireplace, they still get bits and bobs in there, nothing serious, all the good stuff comes from us.