Dressing up is a favorite pastime of ours. Kids get very excited to put together zany outfits with random things they find around the house. Rarely do we refer to it as costumes: it’s just called dress up.
Comparing these two words make me realize that both words have similar meanings: putting on external accessories to appear like someone else. However, the word costume sounds like a deliberate attempt to be like some other specific person or thing whereas dress up seems more informal.
Our dress-up bin contains a variety of accessories: tiger ears, a pirate treasure box, beaded purse to carry to a tea party, butterfly wings, gloves, wands, goggles, hats, sunglasses, etc. They sometimes wear mixing bowls on their heads and march around holding spatulas. Get the wrapping ribbons out, the kids wrap scraps around their wrists to blast power beams and tie around their heads to become so-called ninjas.
To create more possibilities I get tempted to buy more dress-up items, but I consciously stop myself. My rationale is that dress up becomes more spontaneous if ordinary things can be creatively used to imagine ourselves different. But don’t hold me to it because I may just cave when I see dress-up props on sale at local craft stores!!