For Dinner:
We started this tradition about 20 years ago, and I've convinced a lot of friends and family to do the same because it's so fun.
Everyone gets to contribute a menu item to Christmas Eve dinner (enough to share with everyone else) and we all eat all of it. The menu with small kids and big kids has been the most fun. Last year's menu for example:

Brisket
Mashed potatoes and gravy
Cinnamon Toast Crunch
Arby's Roast Beef Sandwiches
Banana Pancakes
Bacon
Cookie Dough Built Puffs
Orange Julius

We also play a game on Christmas Eve where everyone gets a dish of chocolate pudding. We hide one nut in one of the bowls of pudding. The person who gets the nut gets to open a gift that is for the whole family.

We sing carols around the piano.

We do pajamas and read the Night Before Christmas (we actually watch a movie of the poem) and the kids have a big slumber party set up upstairs in a shared bedroom where they all hang out.

They love it. The kids are already building forts in the bedroom upstairs. We are already getting everyone's menu items on the white board in the kitchen (they talk year round about what their menu contribution will be)

I really love the day.

We also do a Christmas package for a family from my mission every year (polynesian country). It's fun to have the kids save up to put that Christmas together for them every year.