Whose idea was it to put a cherry on top of ice cream as the finishing touch?

I think that distinction belongs to an entrepreneur named Chester Platt.

The ice cream sundae was invented by Chester Platt who owned the Platt & Colt's drugstore in 1893. Platt prepared a dish of vanilla ice cream for the Reverend John Scott on a Sunday. Chester Platt spiced up the ice cream with cherry syrup and a candied cherry. Reverend Scott named the dish after the day. An advertisement for "Cherry Sunday" served at the Platt & Colt's drugstore has helped document this claim.

The birth of the sundae goes to Ithaca, New York, which as what is the earliest known documentation via an ad placed by Chester Platt in the Ithaca Daily Journal on April 6, 1892 for the “Cherry Sunday” served at Platt & Colt’s soda fountain.

http://www.thenibble.com/reviews/main/ice-cream/the-history-of-ice-cream4.asp#

http://inventors.about.com/od/foodrelatedinventions/a/Sundae.htm