Type up a letter (if able, if not then just write it) to the Head Chef around the third night thanking him/her for the exquisite meals they had planned. It is very possible that he will invite you to dine with him and/or send you a basket of gifts. You can always find out the head chef's name. If you go to the Guest Relations desk, on one of the walls nearby will be a photo board with the key staff/crew on board. The head chef will be one of those people and his/her name will be listed there. When possible, you should always address letters like this by name.