Decide which part of the Caribbean you'd like to visit. The islands in the Netherlands Antilles tend to be less tourist-oriented because they don't have as many ports for cruise ships.
Think small. The smaller the island, the less tourist activity. Search islands that you might not have heard of, because you never know what they can offer in terms of relaxation.
Choose a date. If you're looking for quiet, book your trip in April and May. This is the end of the cruise ship season and the prices tend to be lower to attract more guests.
Study up on where you're going. Make sure there are no festivals or events happening on the island during the time you're planning on taking your trip. Most festivals tend to happen early and late in the year.