Showing a smile is charity
Allah has promised He will give reward to those who do good deeds and showing a smile is a good deed.
In fact, showing a smile is one form of charity. We are encouraged to smile to our brothers, sisters, friends, and family. We are even encouraged to smile to people we do not know.
The Prophet of Allah 
was reported to be smiling the most. He would always show a smile to the people.
- The Prophet Muhammad  said: "Do not abuse anyone, do not look down upon any good work, and when you speak to your brother, show him a cheerful face."
[Sunan of Abu-Dawood, Hadith 1889]
- The Prophet Muhammad  said: "Charity is prescribed for each descendant of Adam every day the sun rises." He was then asked: "From what do we give charity every day?" The Prophet answered: "The doors of goodness are many; enjoining good, forbidding evil, removing harm from the road, listening to the deaf, leading the blind, guiding one to the object of his need, hurrying with the strength of one's legs to one in sorrow who is asking for help, and supporting the feeble with the strength of one's arms, all of these are charity prescribed for you." He also said: "Your smile for your brother is charity."
[Fiqh-us-Sunnah, Volume 3, Number 98]
- Abdullaah ibn Haarith  said: "I never came across a person who smiled as much as Prophet Muhammad . Prophet Muhammad  regarded smiling to a brother as an act of charity."
- Jarir bin Abdullah  said: The Messenger of Allah  never refused me permission to see him since I embraced Islam and never looked at me except with a smile (on his face).