In the original story, there is a Princess who is able to take her head on and off. She only does so in the privacy of her castle. The boy catches her taking her head off. This is where I would change things up a bit. After the boy catches her doing so, he goes and spreads the word. This is then where I would make things a little more interesting happen to the prince. In the original story the princess dies, and tries to get revenge on the boy for watching and he called her a witch. I think that I wouldn't make her evil, but just have her teach the boy a lesson not to invade other's privacy.
For the beginning of the story, I think I should also talk about why the princesses head is detachable. Was she born that way? Maybe. I feel like it could be some type of legend. Maybe the princess caught someone else doing that so it could end up being some type of spell that is cast upon whoever watches. Maybe the king and queen were evil to the princess in the earlier years of her life.
I also thought may be this could be some type of love story between the princess and the boy. Maybe they somehow end up together after both of their heads are chopped off. I think it could be kind of romantic in a way. They might end up different from everyone else that they know, but at least they have each other.
Bibliography: Russian Fairy Tales by W.R.S. Ralston