To make a jellyfish costume, glue battery-operated color-changing lights to the inside of a plastic salad bowl. Put the battery switch outside of the bowl so the wearer can turn the lights on and off. Cut iridescent cellophane to fit over the lights, and then attach the cellophane to the bowl and the bowl to a helmet. Add light-up ribbons to the outside of the bowl to serve as tentacles, and carry a bubble gun for a comic effect.
Alternatively, open an umbrella, and cover the metal or black parts with white enamel paint or tape. Make the first eight tentacles by cutting large strips of bubble wrap into varying lengths and then taping the strips together to create longer tentacles. Cut the bubble wrap into 16 strips that are 22 inches long and 5 inches wide to create smaller tentacles. Distress the long edges of the tentacles, and then cut eight strips of iridescent white cellophane. Attach the cellophane strips to the top of the umbrella between the shorter tentacles.
Cut more small tentacles and attach them so they hang from the inside of the umbrella. Tape the long tentacles to the cross spokes at the top of the umbrella. Dress in white, and hold the umbrella overhead to complete the costume.