Dunn's River Falls
Arguably the single most popular tourist attraction in all of Jamaica, Dunn's River Falls is a place for both witnessing beauty and having fun. Fed by natural springs, the water cascades down natural staircase-like features from a height of about 200 feet to reach the Caribbean Sea. Tourists, led by guides, start at the beach and wind their way up the falls, holding hands for greater stability. The volume of water is substantial, making the travel somewhere between exhilirating and terrifying, depending on your definition of "adventure". :)
Even if you don't want to walk up the falls, you can still enjoy their beauty from numerous observation points; this was my favorite, with several terraces of the falls framed by decorative ginger plants.