There are a number of nearby islands that can easily be visited in a day. You can go on your own or join a group tour; either option is very reasonably priced. Most people take a boat or ferry, however it is possible to fly. Here are some recommended day trips that can easily be accomplished:
Anguilla
A British island about four or five miles off the French side. The island itself is in square miles the same size as St. Martin. It’s topography is very different. There are no mountains or hills for that matter. It is much dryer than St. Martin. But it does have some of the most beautiful beached in all of the Caribbean. There is no shopping and few good restaurants, but if you like beaches and snorkeling it is well worth the trip. You can take a ferry from the harbor in Marigot to Anguilla, the easiest island to visit, for $12 each way.