Clad in verdant, jungle-covered hills, the twin islands of St. Kitts and Nevis are emerald specks in the Caribbean Sea. On St. Kitts, the dormant, crater-topped cone of Mount Liamuiga rises above old sugar plantations. Its smattering of historical relics is a reminder of the colonial past that shaped the modern Caribbean. Across the channel, Nevis moves to a slower beat—its golden beaches, tranquil villages, and leafy gardens beckoning travelers with a laid-back charm. St. Kitts and Nevi...