To call Santa Clara an "island" surely seems like an euphemism... but if you're in Donosti in summer time and a bit bored, don't hesitate to go there! You have to catch a small ferry on the port which leaves every 30 minutes; the ride only takes a few minutes. Being on the island you can see the whole Bahía of La Concha (and Ondaretta) perfectly. There's not much you can do there: take a look around, enjoy the sunshine, maybe take a swim and that's about it but it's still pretty enjoyable.
Ferry service: june 1 - september 30; 10:00 - 20:30
Round trip: 3,60 €