I have a notebook that I keep for playing at receptions and things - it is mostly things I have played before that won't require much prep time. I use a lot of the Mendelssohn Songs Without Words, classical sonata slow movements, etc.
Something else that's easy that most people would enjoy as background music is some of the intermediate pedagogical teaching literature. Stuff like Martha Mier, Dennis Alexander, Robert Vandall, Catherine Rollin, etc. I know some people here are "classical only," but this is easy to sight-read, patterned, and a lot of it has a nice sound to it. (Be warned though, some of it is cheesy and definitely "kid-centered.")