Porto Cervo is small enough to walk end-to-end in twenty minutes, but the beaches and the rest of the Costa Smeralda are spread over a stretch of coast that effectively requires a car. There is no useful public transport between the resort and the surrounding bays.
By car. A rental is by far the easiest option and is best picked up at Olbia airport on arrival. The SP59 is the main road through the Costa Smeralda; smaller spur roads lead to each of the beaches. Parking in Porto Cervo itself is paid and limited in season; the beaches have their own car parks, most of which fill by mid-morning in August.
By boat. Many visitors arrive by their own yacht or charter a small motorboat for the day from the marina. A skippered rib opens up beaches you cannot reach by road — Cala Liscia Ruja, the islet of Soffi, and the southern beaches of the Costa Smeralda — and is a far more pleasant way to see the coast.
Taxis. Available but expensive; expect resort rates and pre-book in high season. Some hotels run shuttles to specific beaches.
Bike. Possible but rarely sensible: the roads have no cycle lanes and the granite hills are steeper than they look.