La Cerdanya is a roughly two-hour drive from Barcelona. Take E-9 to the Túnel de Cadí (Cadi Tunnel). Once you're through the tunnel, follow signs to N-260. Routes N-260 (Spain) and N-116 (France) bisect the valley. Pick up a Michelin map for main roads, and a regional map, such as Editorial Alpina's Mapa Excursionista y Turístico (editorialalpina.com), for smaller mountain roads and scenic country byways. Border patrols in the adjoining towns Puigcerdá (Spain) and Bourg-Madame (France) rarely stop cars, but it's a good idea to have passports ready.
For best rates, rent a car prior to departure via Auto Europe (888/223-5555, autoeurope.com). In September, midsize car rentals average $400 per week for a manual transmission with basic insurance, and $850 per week for an automatic.