The Cíes Islands are three small granite islands off the mouth of the Ría de Vigo, in the south-western corner of Galicia — one of the most spectacular natural places in Spain, and the centrepiece of the Illas Atlánticas National Park. The British newspaper The Guardian once called the long curved beach connecting the two northern islands, Praia de Rodas, the most beautiful beach in the world. That is the kind of claim that normally deserves a raised eyebrow; here it is genuinely defensible. The sand is powder-fine, the water is a shocking turquoise, and behind the beach a shallow lagoon is filled by the incoming tide.
The islands have been protected since 1980 and are now subject to strict visitor quotas — only around 1,800 people are allowed ashore per day in summer, and you need a permit (available free online through the Xunta de Galicia's website) before you can buy a ferry ticket. That cap is what keeps the place from being overrun. Once you step off the boat, the islands are entirely car-free: a network of walking paths leads to the lighthouse on Monte Faro (the highest point, about 175 metres, with a 360-degree view that on clear days reaches to Portugal), to the seabird observatories on the clifftops, and to a handful of quieter beaches on the western side. The cliffs on the Atlantic-facing side are one of the most important breeding colonies for yellow-legged gulls, shags, and common murres in Europe, and the noise during spring nesting is astonishing.
There is a small campsite on the middle island if you want to stay overnight (reservations required, and book well ahead for July and August), a single bar-restaurant at the landing area, and nothing else — no shops, no hotels, no fresh water beyond the taps near the beach. Bring everything you need for the day. Boats leave regularly in summer (roughly May to October) from Vigo, Baiona, and Cangas, and the crossing takes about 45 minutes. In winter the islands are closed to visitors.
Combine a day on the Cíes with a meal of seafood in Vigo — the mussel rafts visible from the boat on the way out supply much of what you'll eat — and you have one of the best days of travel available anywhere on the Iberian Peninsula.