It’s worth remembering that the monks on Athos are expecting religious pilgrims, not tourists. Each monastery varies a good deal in their handling of visitors; signs at some institutions specifically forbid you from attending services or sharing meals with the monks. Others put themselves at the disposal of visitors of whatever creed (though you may well find yourself being encouraged to switch to Greek Orthodoxy!). Yet, wherever you stay there’s a certain amount of etiquette to observe.
You must reach the monastery where you will overnight before dark, since they all lock their front gates at sunset – which would leave you outside with the wild boars. Upon arrival you should ask for the guestmaster (arhondáris), who will proffer the traditional welcome of a tsípouro, loukoúmi (Turkish delight) and often a Greek coffee, before showing you to your bed. Most guestmasters speak good English.
You should be fully dressed at all times, even when going from dormitory to bathroom; shorts should not be worn anywhere on the peninsula, nor hats inside monasteries. Swimming is officially prohibited, so if you are tempted, choose a cove where nobody can see you, and certainly don’t skinny-dip. Smoking is forbidden, though it is often tolerated outside the monastery walls; it would be criminal to smoke on the trail, however, given the chronic fire danger. Singing, whistling and raised voices are taboo; as is standing with your hands behind your back or in your pockets. If you want to photograph monks you should always ask permission, though photography is forbidden altogether in many monasteries, and video cameras are completely banned from the Mountain.