Going on a cruise need help with excursions any suggestions?

Ok I am going on a seven day cruise for my honeymoon and I want to do stuff but I am really worried about our safety and also we already spent so much on the room we need cheap fun things. We are going to Grand Cayman, Rotan, Belize, and Cozumel. Does anyone know some shore excursions that are fun and on the cheap? has anyone gone and done things that were a DO NOT MISS... tell me what you think.