Volunteer your time. There are an infinite number of organizations or services that would love to have you spend a few hours helping them, so pick one and pay a visit.
Think of what you would like to do to help the organization and feel good about yourself at the same time. Is it sitting with a few elderly people and listening to them tell you about their lives? Delivering supplies for a homeless shelter in your community? Or answering the phone for a local fundraising project? At any given moment, a project that you would like to see become more effective could use your help. If you only have an hour to give, then give an hour. If you can devote some time on a regular basis, your cause would find that most welcome.
When you volunteer, think about how much pleasure you are deriving from your efforts and how much they are appreciated. Volunteer at the same place, or spread your time around many causes, whichever feels right.— Alan Epstein in How to Be Happier Day by Day