Hug every snowman, not counting dreamworld ones (meaning the meta-snowmen that you make inside the dreamworld, where there are no walls).
For me, I did this in the second maze that you get to after going through the south gate once you make every initial snowman. I assume you can do this in the initial maze, but I haven't tested. I went through and hugged every snowman for a few seconds. Try to be systematic about it because I don't think there's any way of knowing if you've hugged a particular snowman or not. If you're doing this in the second maze like I did, don't forget about the three that are north of the second maze, where you learned about building meta-snowmen in the dreamworld.