I would get two of the same brand of soprano in their favorite colours. The attached card could make reference to marriage being a duet rather than two solos. May these ukuleles help bring harmony to your marriage. May you learn how to both be in the same key--on the same song--on the same page-- these things take practice. May you from this day forward say, "I have yet to be able to_____ rather than say, 'I can't do _____. " You are not getting married to create a past or a present, but a future together. Togetherness is the essence of a good marriage. Your intent is to give them a way to experience it in a non threatening scenario. They have yet to discover what else they will enjoy doing together as time goes on. The advice which goes along with it can show your desire to help them do new things together. It certainly is not to be considered as a gag gift. You are to be commended for thinking outside of the box. Anyone can give cash.