Roatan lies off the coast of Honduras and is the largest of the Bay Islands. It is a fairly small island, about 37 miles long 5 miles wide. In the course of a day you can easily make it all around the island.
Many parts of Roatan are covered with lush vegetation and it has a nice varried terrain. Unlike many small islands, Roatan is not flat, although I wouldn't call it mountainous either.
Roatan is known for having great scuba diving and snorkeling right off the island. In several spots you can walk right off the beach and right into prime snorkeling and scuba diving areas with coral reefs and a wide variety of sea life.
Just in the last few years a cruise port was built and now most cruise lines stop at Roatan on some itineratry. Roatan doesn't have a lot of public transportation, but for around $70 you can hire one of the many drivers that will line the street outside the port. They will drive you around and stay with you, and be your personal tour guides for the day. They will know some good places to go and I'd recommend bringing along your own gear if you plan on snorkeling.