I always make advent calendars for the kids and put fun stuff in, more like stockings as we prefer to spread the fun of small gifts over the month than on one day which is also has big gifts.
i have teen girls and there advent gifts are easy: travel size bits of make up, lip balm, perfume sample bottles, underwear, nice soaps etc.
when my son was younger I did mini dinosaurs, some years I split Lego packs so he could build a Lego set a bit each day, fidgit toys etc. now he’s not little anymore I am really stuck for ideas. Last year I did football badges and stickers for his team. I have no idea this year. Any suggestions gratefully received.