Having a daughter (or more), you probably have a few dolls laying around of the plastic Mattel variety. Maybe you even have half of one (hopefully the top half).
Brush up your fishtail braiding skills to create this Mermaid Tail style. Start it loosely so you can stuff Barbie at the top, and give her a shirt.
I really think splitting the end into a fishtail completes our ensemble, along with the green hair coloring.