My oldest daughter decided that she wanted to be Spiderman for Halloween a long time ago, like months. For a three-year-old, that is a very long time. Luckily, she hasn't changed her mind about it either. I don't know why she is so into Spiderman, except for maybe some fatherly influences, but she is, and so we are going with it.
We tried on a cousin's old costume with built-in muscles, and it just looked weird on a little girl with long, blonde hair. Then, I saw this on Pinterest: