The classic version sees you handcuff the stag to a lamppost and then leave him there. The ideal location for this is right outside the bar where the rest of you are sitting and enjoying a good drink.
You can extend this with a sign around his neck - "Spank Me" always works a treat. Alterntively, hang the key around his neck and tell him to unlock himself using his teeth.
Dress the stag up or down but DO remember that some passers by are amused by wit but not by rudeness or crudity. Pouring drinks or baked beans over his head can slightly ruin the evening for him or get him excluded from later pubs and clubs.