Mountaineering is prohibited in Bhutan because the people believe mountains are the abode of gods and allowing climbing would defile the environment, which would earn the wrath of the divine in form of disease and poor harvest.
Lakes are also considered sacred with deities living in their depths. Visitors are cautioned not to throw anything in the lakes that would pollute its sanctity.
Always go in a clockwise direction around monasteries, temples, stupas and mani walls. When entering a temple always take off your shoes. Have some small change ready to make an offering at the altar, which is customary practice in Bhutan.
Dress modestly, especially in villages and temples.
If on a trek please collect your personal trash and give it to the staff in the morning for proper disposal.
Take your old torch batteries home with you.
In general the Bhutanese are not shy about having their picture taken, but ask for permission first.
There is a national code for the etiquette of Bhutanese people called Driglam Namzha.