As above, weather very unpredicatable.
Other possible days out – Use the metro to go to Las Arenas and see the hanging bridge. Have a little walk along the river. further along the metro you have Plentzia, a nice coastal town, go walking and watch the surfers. Nice little walk up to the Lighthouse for good views.
If you go to Gernika have a look inside the Basque parliament, nice little building and the museum there is ok.
Day trip to San Sebastian is doable.
If weather is good, theres a nice, easy walk from Apatamonasterio that takes you under Anboto, the bar at the end of the path is “Etxebarri” which was in “The trip, Spain” – you can just go in for coffee and pintxo.