3d Christmas tree
Advent calendar - I'm making one this year which will have things like stickers, hair clips, hand painted wooden peg dolls, little chocolate occasionally etc.
Fairy lights make things look special
Visiting a Santa's grotto
Ice skating
Going to buy a special ornament for the tree and helping decorate
Salt dough decorations they can make and paint themselves
I just did baked orange slice garlands and my nearly 3yr old loved that
Could also do popcorn garlands or paper chains
Window stickers like you said and a wreath on the door or little reindeer ornaments etc
Going on a winter walk... My dd likes going out for a night time adventure where we wrap up and look for the moon/owls/bats
Have a look on Pinterest for family traditions to start - matching pj's, Xmas eve movie night or watching a Christmas film each weekend leading up
Little Christmas tree in their room or a pompom wreath on their door or paint a santa and add cotton wool beard to stick on their doors
Stick any nursery or school Xmas arts up on the fridge etc.
The works have lots of Christmassy bits cheap - colouring books, kits, card making
Going round where you live to light switch ons and taking photos to put on the wall or in an album
I'm sewing a christmas dress for my dds favourite soft toy
If they have a dolls house or small world play you could decorate that like little bristle trees or putting tinsel up etc.
Make a gingerbread house and display it or put a jar with gingerbread men or Xmas treats in
Cut out shapes in sandwiches like a snowman or star
Themed play ideas like shaving foam and glitter etc
I know some of these aren't home decor but I find the run up can really excite children and you can take photos of these things or stick art up and they can look at them or older kids can make a Christmas scrap book. Can be helpful for a transition so they can recall happy memories