San Sebastian is your beach/surfer spot to chill and process al the things you saw during your trip.
Pension Goiko, was the place we stayed at, and honestly it didnt really leave an impression. Meaning it wasnt really good, but also not a horrible experience either.
If there are a lot of things to do in this place, i didnt notice. After a many train rides and sightseeing trips, my friends and I just relaxed, watching the sunset, going to the beach, and walking through the city. The atmosphere of the place is great and I am sure that if you have the chance to stay longer, it is a great place to go exploring.
Going back you can either take a train to Barcelona or Madrid to fly back, or cross the Border to France. If you have more time, Bilbao could be a great extension of your stay.
Also the budget was around 25 Euros a day, but it is hard to tell since this istn the only country I went to with my Interrail ticket.
Other than that, i hope you have a great Trip! I for sure did!
(and please forvige me typos and spelling errors, english is not my first lenguage)