From San Sebastián · 43.32° N · 1.98° W
One- or two-night getaways from San Sebastián for 1 May: the Arcachon bay in its month, Llanes, the Mundaka estuary and the Bidart path with a boat back.
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April and May are, with September and October, Arcachon's best months: the Pilat dune without summer's crowds and the oyster farmers' huts open, with a dozen at €10 or €13. Check the tides before booking the boat: at low tide departures change.

In May the Llanes coast is green and its beaches almost empty, because the festivals and the crowds arrive in July. Kayaking between coves and caves depends on the sea: book it and let the company give you the time according to tide and swell.

May to September is Urdaibai's season, and May has it without August on top. Go up to Gaztelugatxe first thing and check beforehand whether that day is controlled access, as set by the Bizkaia provincial council. The estuary canoe goes out according to the tide.

Spring is the best time for the 13 km of coastal path from Bidart to Saint-Jean-de-Luz, with temperatures right for the climbs and the cliff boat running since April. In summer there is almost no shade; in May you walk happily all day.

The wooden La Rhune train runs from the end of March, and May is among its best months. In France 1 and 8 May are holidays: those days the queue is longer, so buy a timed ticket on its website and go up early.
April, May and June are the best time for the Romanesque of northern Palencia, with the moors green. The Convento de Mave serves lunches from late March to October: order the roast lamb when you book. The Sunday European bison visit is also booked.

May is one of Olite's marked months, with the Zona Media countryside green and just enough warmth to climb the Royal Palace towers. An hour and a half by motorway and back for dinner. The grill on the Rúa Mayor is closed Tuesday and Wednesday.

For those with four days: from April to June the Añisclo canyon is full of water and the road is quiet in the morning. From San Sebastián it is a little over three hours, so it is better with two nights in Aínsa and the Guara canyon booked.
Questions
Yes. It is one of its best months: dune without queues, oyster huts open and mid-season prices. The water is still cold for swimming.
1 May and 8 May, besides Ascension and Whit Monday, which change date each year. On those days the French coast has more people.
The Bidart to Saint-Jean-de-Luz path, La Rhune with Espelette or Olite and Ujué. All three let you be back for dinner.
Only on controlled-access days, set by the Bizkaia provincial council's annual calendar. Check it before going.
Geocaliahttps://www.geocalia.com/en/spain/basque-country/san-sebastian/getaways/may-long-weekend