An enhancement; a bonus: going out to dinner and a movie is icing on the cake
The Dictionary of American Slang, Fourth Edition by Barbara Ann Kipfer, PhD. and Robert L. Chapman, Ph.D. Copyright (C) 2007 by HarperCollins Publishers. Cite This Source
Idioms and Phrases with icing on the cake
frosting on the cake. An additional benefit to something already good. For example, All these letters of congratulation are icing on the cake, or After that beautiful sunrise, the rainbow is just frosting on the cake. This metaphoric expression alludes to the sweet creamy coating used to enhance a cake.