The Bosphorus, Istanbul, Turkey

The Bosphorus is the 32 km (20-mi)-long strait which joins the Sea of Marmara with the Black Sea in Istanbul, and separates the continents of Europe and Asia. It's great for a half-day cruise north toward the Black Sea. You can return to Istanbul by land along the European shore and see all the sights. It runs right through the heart of Istanbul, past the Istanbul Modern Art Museum, several Ottoman palaces, at least two fortresses, forested hills, and shore villages with Ottoman architecture. (For self-guided touring, I've divided it into the Southern Bosphorus and Northern Bosphorus.)