Recommedations of excursions on a Westerm Carribean Cruise?

Hi, we are cruising on the Carnival-Valor to Grand Cayman, Roatan, Belize & Cozumel. We booked a dolphin swim in Cozumel and snorkeling with stingrays in Grand Cayman. We don't really want to spend much more money as it is already costing a fortune but wondering what we can do in Roatan and Belize. Anything that is a must see or that is a must do? Thanks,