The city itself is one bay you can walk around in an hour, so the good days are half beach and half the things Eduardo Chillida left behind at either end of it.
Playa de la Concha and its promenade
The shell-shaped bay and the white railed promenade that defines the city. It is free, it is in the middle of town, and the tide changes it completely — at high water there is very little sand left at the Ondarreta end.
Monte Igueldo funicular
The 1912 funicular at the western end of the bay. €5.50 return for adults and €2.50 for children up to 7, and the ticket includes entry to the small old-fashioned amusement park at the top. Note they do not sell tickets for the descent only.
Peine del Viento
Chillida's three rusted steel claws bolted into the rocks past Ondarreta, at the far west end of the promenade. Free, always open, and best in a swell when the blowholes in the terrace under your feet start firing.
Chillida Leku, Hernani
The sculptor's own farmhouse and meadow, with monumental steel and granite pieces set among the trees. €14 full, €10 reduced, free under 8; open Monday and Thursday to Saturday 10:00–19:00 and closed Tuesday and Wednesday. The BU05 bus stops at the museum.
Aquarium Donostia
At the end of the harbour in the Parte Vieja, Plaza de Carlos Blasco Imaz 1. Half of it is Basque maritime history — whaling, boatbuilding — and half is tanks, ending in a walk-through tunnel. The obvious wet-afternoon option, and it is right where you would be walking anyway.