Rock en Seine has grown from a 2003 upstart into one of France's biggest mega-festivals without leaving the same park: the Domaine National de Saint-Cloud, 1,100 acres of formal gardens and forest just outside Paris, five stages spread across five days. More than 100,000 people show up over the run. Single-day tickets start from around 79 euros, with multi-day passes climbing toward 230 for the full stretch. Bring good shoes; the park is bigger than it looks on the map.