Any vacation off the beaten path can be invigorating, but an eco-tourism volunteer trip adds a layer of beauty and purpose. Participants trade work—rebuilding hiking trails, tracking sea turtles—for a natural landscape, a community of like-minded travelers, and the satisfaction of helping the planet. The best eco-tourism trips start with an honest inventory: how much time and money you can spend, what impact you hope to have, and whether you like sleeping under the stars or under 400-thread-count sheets. "Knowing yourself is key to designing a meaningful trip," says Doug Cutchins, coauthor of Volunteer Vacations. Here are six ideas to consider if you love...