Within the limits of what he is willing to do, a Princess can have her Knight be naked whenever she wishes, and if his limits include public places where nudity is allowed, then she can tell him to be naked.
The dynamic of him being naked because she told him to be can be enhanced by her keeping her bottom on.
For anyone who is used to being naked on a beach this might seem no big deal. But if he has never been naked on a crowded beach before it is likely that this little walk along the beach will feel very significant. What fun!