Heidelberg is the German city that visitors fall in love with. It sits where the Neckar river emerges from the wooded hills of the Odenwald into the Rhine plain, and the combination of the ruined castle on the hillside, the baroque old town below, the river, the ancient university, and the forested slopes rising on both banks makes it one of the most naturally beautiful urban settings in Europe. Remarkably, it survived the Second World War almost completely intact — the Americans deliberately spared it — which means the old town you see today is genuinely old, not a reconstruction.
The Schloss (castle), half-ruined since the French blew it up in 1693 during the Nine Years' War, is the most visited ruin in Germany. Take the funicular from the Kornmarkt to reach it, and spend time in the courtyards: the Renaissance Ottheinrichsbau façade, the enormous Großes Fass (Great Barrel, a wine cask holding 220,000 litres), and the views down over the red rooftops and the river are the highlights. The Alte Brücke (Old Bridge), the 18th-century sandstone bridge with its twin towers and brass monkey statue, is the postcard view of the city — walk across to the south bank and up the Philosophenweg (Philosophers' Walk), the path along the opposite hillside that Hegel, Goethe, and Mark Twain all walked for the panorama back across the river to the castle.
The University of Heidelberg, founded in 1386, is the oldest in Germany and still sets the tone of the city: 30,000 students, packed cafés, and a lively bar scene that runs late despite the tourist-town veneer. The old Studentenkarzer (student prison), where unruly students were locked up as recently as 1914, is a small museum covered in graffiti left by former inmates. The Hauptstraße, the long pedestrian spine of the old town, runs from the Bismarckplatz to the Marktplatz and is lined with shops, bookstores, and restaurants — it gets crowded with tour groups in summer, but the side streets off it are quieter and often more rewarding.
Heidelberg is an easy day trip from Frankfurt (one hour by train) or a stop on the way south to the Black Forest or Lake Constance. Come in the evening if you can: the castle is illuminated three times a year (in June, July, and September) with fireworks over the river, recreating the French destruction of 1693 — a tradition that has been running since 1830.