Nothing.
I'll ride my bike, play guitar and walk the dog. Just like any other day. Might wander down to the Christmas church service.
All my family are either long gone or I have nothing to do with them. I've not actually had a Christmas present in 20 years, although I have bought them for ex Hubs' family.
The chap that I am seeing will be off somewhere with his elderly parents and his son, so I won't see him. However, I will be at my new job by then so I might try and work over the Christmas period. I do find it very lonely. I have volunteered, in the past, as a server at the Salvation Army Christmas lunches. I might see about doing the same at my nearest one here.

It's a lot of fun.
I have started my own personal tradition of a bike ride on Christmas day. I've done that for about 8 or 9 years now. Roads are quiet and I can enjoy the peace.