Nepal can be quite an interesting country to visit due to its unique and sometimes confusing customs. Locals are often very friendly and might approach you with questions, goods they are selling, and cameras to take pictures with you.
One thing foreigners should be aware of is how to say "yes" and "no" to strangers who want something from you. The gesture for "no" is one hand extended with the palm forwards. Do not shake your head. Why? Because it is similar to the gesture for "yes": a sideways tilt of the head from one side to the other.
The forehead is the most sacred part of the body in Nepal. It is therefore considered rude to touch an adult Nepali's head. Many Hindu temples are off-limits to nonbelievers, so be appreciative if you are given the chance to enter one. Be respectful of the customs: take your shoes off before entering, and do not take any photos unless you have been given clear permission.
Most locals wear very conservative clothing, so be aware of how your clothing may appear to them. Men should always wear shirts in public, and they should try to wear long pants, especially when entering a monastery or temple.
There are many customs in Nepal related to food, and these are very important to follow. If you are eating something with your hands, do so with your right hand only, as the left hand is only used for washing after using the restroom. You should try to give and receive anything and everything with only your right hand. It is also important to remember that the people of Nepal do not eat beef. The cow is the national animal of Nepal and is sacred in the Hindu religion.