Here's my idea. Father and his fourteen year old daughter switch bodies and have to spend a day like that. He has to go to school in her body. She has to go to work in his body. Neither knowing that the other secretly has the hots for the other, mom having died years before. The way they get their bodies back is by having sex with each other.
I started trying to write this myself but can't quite pull it off. Maybe someone can take the idea and run with it.