Advent boxes arrive after the Advent procession service at church. They have the seasonal stuff that is used for the whole of December. They have the advent calendars (the oldest of which was mine as a child), advent candles, a nativity set, some Christmas books, some scented oil and candles, a letter from the elf, the elf, and some hot chocolate/mulled wine/mince pies.
The elf brings the supplies for other seasonal activities each weekend in December: making a wreath, baking, making and writing cards, decorating the tree.
When the children were little, the Christmas Eve box was about encouraging children to go to bed calmly. Cosy pyjamas (re-used for anyone who's the same size as previous years) stockings to hang, thank-you cards so children can write their thanks on Christmas day, treats for Santa and the reindeer, more hot drinks and mince pies. There used to be bath bombs or bubble bath, but the children prefer showers now, so some sort of nice shower gel and maybe a pillow spray. I would take them out for a walk to tire them out, and the box would be there when they got back home.
Now they are teenagers, and the box has more stuff about preparing for Christmas day together, so we still go for a walk, but the box now has vegetable peelers, ready for a Christmas Eve food-prepping session.