We have visited Donostia San Sebastian many, many times and stayed in many, many hotels and Airbnbs. This was genuinely the best positioned in terms of access to the Old Town, the new town, parking, the port and the yacht club (so important for that sundowner G&T!).
Leaving the car in Parking Boulevard was about EUR20 a night and, if you emerge from the eastern flight of stairs, less than 2 mins walk to the flat.
Within 20m of the flat there are two bars, a drugstore, a general store, a launderette, a fish shop and a specialist food shop. What more could you wish for?! 100% recommend.