Ons is best enjoyed slowly, on foot. The island is small enough to walk in a single day — about six kilometres from the pier at Area dos Cans to the lighthouse at the northern tip — but rewarding enough that you'll want to linger. Visitor numbers are capped by the national park, which keeps even July and August from feeling overrun.
The western Atlantic coast is the dramatic side: sheer cliffs, booming sea caves, and the white tower of the lighthouse above it all. The eastern side, sheltered inside the Ría de Pontevedra, is calmer and sunnier, with the beaches that most day-trippers come for. The marked trail network connects everything; the main loop takes three to four hours at a comfortable pace.