Please do try to follow a stretch of the Camino de Santiago if you enjoy walking - depending where you are staying you will have no problems finding a stretch and following the footsteps of a thousand years of pilgrims is reward in itself but the route goes through great countryside and villages and there are some excellant hostelries along the way. You can join an organised walk or just plan a couple of days for yourself and it is a fantastic way to get a feel for the region.
Vitoria is a pleasant city and the official capital of the region with a cathedral worth a visit. Do go there because most people don't but San Sebastian is my favourite. Spend time in the Parte Vieja for restaurants and bars but make sure you get up into the hills for the view across the Bay of La Concha especially at sunset. If you go to Guernica be prepared to be disappointed although there are a couple of nice sculptures including a Henry Moore in the park and a dead oak tree (you will have to go to Madrid to see Picasso's masterpiece).
Above all things enjoy the food and do not leave without trying some of the amazing iberica ham and a bocadillo tortilla de patatas, the best hand snack known to man! And churros of course