Marvelous idea for a birthday party activity, using sheets of craft foam from the craft store and whatever embellishments catch your eye while you’re there.
Younger children will need models like here in the graphic and on the page referenced above. Older kids can be challenged to create the most outlandish hats with the materials you provide. Feathers (large and small), paper flowers (large and small), fabric scraps, crepe paper, whatever you can think of. The idea here is actually NOT to make it easy- but rather to make it a challenge. This is no doubt more of a girl-thing than for boys.