The Summer I Saved the World...in 65 Days by Michele Weber Hurwitz

Nina's having a lost summer. Her parents are working all the time, her brother is too busy for her, and her best friend is busy  with things that don't interest Nina. But then Nina comes up with a plan to do one good thing for someone in her neighborhood each day of summer. Nina has no idea what surprises and secrets she will discover as she tries to save the world in 65 days.