There really isn't much that you can't change in a photo. Change the color of your hair, add a car, switch your face with someone else's, change the color of your car, remove blemishes, etc... The limit is really your imagination. One of the great things about photo-manipulation is that the laws of physics don't apply.
The limit of what you can change in the picture is often limited by the picture itself. If you have to remove something, and you have no frame of reference to re-create, it can be very difficult. If you have something in mind, contact me and lets take a look together.