You should tip the person who brings any room service if you should order any. Usually just a few dollars depending on how much your ordered.
When you go on shore excursions, its always nice to tip the guide if they were helpful and made the excursion enjoyable, sometimes the guy that drives the bus. He or she may also be a guide and narriate along the way.
Don't forget the porters at the pier when you hand them your checked luggage.