If the book is the night before Christmas I would keep it. It's likely a tradition from their mom, rather than GPs. I bought DD a personalised copy for her first Xmas and would want to read that with her regardless of where we do Xmas. Obviously you could add in more books as well.
The same for the pyjamas. I'm guessing it's new pyjamas, rather than the same ones. Most likely so they look nice and clean in any photos or videos taken Christmas Day! I'm guessing their parents will bring those though.
Apart from keeping those I'd say do your own thing, how about going to the cinema? Keeps little kids occupied for a good few hours on Xmas eve when they can be so unsettled. There's the new grinch film out if they haven't yet seen it. And the new Mary poppins will be out the week before.
Baking a good idea but also choosing or making a special Christmas Eve meal that they can have every time they have Christmas at yours (be prepared to eat whatever they choose, maybe give options). We have a particular takeaway every Christmas Eve.
Tracking santa is brilliant, and I love the idea of learning how to say merry Christmas in the language of the different countries, I may steal that for future christmases with DD. With older kids you could learn about Christmas traditions in those countries as well.
But to be honest, nothing beats a movie in Christmas pyjamas, nice food and hot chocolate on Christmas Eve!