Coimbra is definitely worth a visit.
It is one of the oldest Universities in Europe with roots dating back to the 1200's.
The student life melds with the fabric of the city
Its location on the Mondego River has been historically critical and there have been settlements there or near there since roman time.
For some more info have a look at this recent article:
http://barlaventostravelblog.blogspot.pt/2013/01/coimbra-portugal.html
There are plenty of low-cost pensions in Coimbra.
There is also a "Pousada de Juventude", state youth hostel.
See also: http://www.turismodecoimbra.pt/index.php?lang=en
As regards travelling to San Sebastian/Donastia from Porto you basically have two routes:
1) Through Galicia via Vigo, Santiago da Compostela, La Coruna, Oviedo, Santander and Bilbao.
2) Via Salamanca, Valladolid, Burgos, Vitoria, and Bilbao.
This can all be done by long distance coach (See: alsa.es) or train. (See renfe.es / www.cp.pt)
On both routes, there are historic cities worth visiting. In Bilbao you also have the Guggenheim.
There are other cities in the region like Zamora, Ourense, and Leon among others that might interest you.
Basically you need to have a look at the articles on these cities on wikipedia. Decide which ones interest you and then plan your itinerary from there.
