The two used the `` smiley face '' on buttons as well to decorate coffee mugs , t-shirts , bumper stickers and many other things . They also used the phrase `` Have a happy day '' ( which was the idea of Gyula Bogar ) . The two produced buttons as well as coffee mugs , t-shirts , bumper stickers and many other items emblazoned with the symbol and the phrase `` Have a happy day '' ( devised by Gyula Bogar ) which mutated into `` have a nice day '' .