Well, if you have a car I´d suggest to stay in two places. Hondarribia is a very good choice, as it´s well linked to San Sebastian and the French Basque Country, and the beautiful inland towns south of SS. Another choice close to San Sebastian could be Getaria (great restaurants, great wine and vineyards, cozy town) or Zarautz. In the other province, and close to Bilbao I´d recommend Plentzia or maybe Mundaka, both beautiful towns and well linked to the rest of the area. Not Lekeitio, parking is a hassle and a very curvy route anywhere (but of course, don´t miss it, it´s gorgeous). About the weather: very unpredictable, it may rain any time (there´s no "rainy" season over here, it´s just a rainy area), so be prepared for anything (even very sunny days...but not too warm, though, May should be in the 70s)