Some times as you are ready to depart from your overnight accommodation and expect the bill, the food charges will be there but not the accommodation charge. Instead you may be asked to pay as you wish.
My feeling is that this may sometimes be a competition factor, but there have also been times when there were no other lodge option in the vicinity. I guess it eventually comes down to a strong sense of duty and hospitality - and it is important not to abuse this.
My tip is that you offer to pay what the going rate would be for a similar lodge and service in a similar place. And add some extras.
Gosainkund is a serious place of pilgrimage (more on that in another tip later), and the goal is to come and do "darshan" and rinse oneself in the icy waters of the Gosainkund lake. This pilgrimage is significant for Hindus and Buddhists alike. If you know someone in Kathmandu or elsewhere who you'd think would like to have some holy water, bring some back. There is such reverence and emotion over such a gift - don't miss to contribute this! Most likely the religious head of the family will take care of the water, give out a schluk for the family members, engage in puja and sprinkle the house with it.
Most people in the Langtang region are Tamang and they have their own language. Even though the majority of people will understand Nepali it is always nice to learn just a few words of Tamang.