A resident of Isla Mujeres told me that one of the things he loves about the island is that everyone is happy. And it’s a good point — with the warm sun and the beautiful blue Caribbean views in every direction, it’s hard not to be in a good mood. On top of the sunshine and scenery, the island also has an unquestionably relaxed vibe, full of visitors lounging by the beach and riding around in golf carts. . . .