Fill the balloon with water . This image illustrates why you don't completely fill the balloon you should always leave some room in the neck of the balloon, then as the form dries the pressure has somewhere to go and your clay wont split like that.
Close up skull would be a good tattoo, with something hidden in the eye. city scape, comic book scenario/villain, doctor who reference etc. Thigh/side/leg/upper right arm (ribs maybe? but PAINFUL)