Are you working on a novel? Or do you have a favorite character from a novel by another writer? This is an exercise that requires an established character, one that you know really well, although it does not need to be one you created.
Pick a character and then write down his or her name at the top of a sheet of paper. If your character is male write a female version of his name, for example Henry Lee would become Henrietta Lee. Not all names can change gender so easily, so if you have to modify it a little more go ahead. If your character is female write down the male version of her name.
Your main character for this writing prompt is the same character that you initially chose. The only thing that has changed is their gender.
Now that you have a name and a character, you have to do something with them. If you have already written most of your book, or if you have taken your character from an already existing text, you should probably take an existing scene and re-write it with the main character now having the opposite gender. Or you can write a scene from scratch.
There is a lot of room for a flexibility. You can take this prompt in a comedic direction or a serious one. It is all up to you.
Happy Writing!