Golden tablets with the seal of the ruler that issued the pass and the seal and name of who it was issued to. About the size of your palm of solid intricately etched gold. The holder of the tablet is backed by the authority of the king to make any required purchases on credit or enter any place they deem necessary. A very powerful token indeed.
King dresses as a commoner occasionaly and makes his way through the town. It is an unknown occurance so nobody expects it. This time he is mistaken for a thief and jailed. Not wanted to reveal himself, and always curious for the experience, he allows himself to be arrested. PCs also get arrested for something, probably guilty but pay a fine. How they act towards the 'king' could greatly change how their first meeting will be as the players always meet the king at some point.