The capital of French Guiana is an agreeable town with weathered Creole and colonial architecture. Throw in Cayenne’s museums and a buzzing market and there’s enough here to keep you engaged for a day or two. But the true marvels are offshore at islands that used to be France’s most feared penal colonies, or a short way down the coast where exotic snakes, caimans and spiders make their habitats in the Kaw swamp.