Halloween usually calls for frightening, creepy costumes, like skeleton makeup or Pennywise clown looks, but this year, step out of the box with something more creative like a pop-art costume.

As accidental adverts for art shows go, a giant pooch made of flowers is a crowd pleaser. Outside the Guggenheim Bilbao in northern Spain, Jeff Koons’ much-loved flower 1992 sculpture “Puppy,” shows ...