This person put a windvane rooster on top of theirs. And this article suggests:
An example of something you collect, such as three Beanie Babies tied together with a big bow,
Something you like, such as a dragon,
A reminder of someplace you have traveled, such as a model of the Eiffel Tower,
A representation of something you like to do, such as a motorcycle model if you ride or crossed mixing spoons if you cook or silk flowers if you garden,
An example of your favorite animal or mythological being, such as a lion, a cat, a phoenix, a fairy, or a dinosaur,
Or even a family member, possibly a laminated and trimmed photo of you and your wife.
Basically, you can put anything that fits and doesn’t weigh too much on top of your tree. Peacock feathers, glitter-covered pine cones, a big piece of gaudy jewelry, a decorated sugar cookie, a snow man made out of styrofoam balls, a piece of plastic fruit or a Carmen Miranda-type mass of fake fruit, a round ball in whatever color seems to sum up your tree, an origami creation made out of shiny paper, a favorite hat, a pretty scarf, a fancy sock, a glittery evening glove, just about anything you want.
This person made a felt Cthulhu to go on top of their tree. You could modify it to a jellyfish or even an octopus if you want.