San Sebastian, Basque Country: 8-24 hours is not enough
San Sebastian is one of those places that warrants you playing hooky from other things you may need to do in Spain. Go ahead, you deserve it.
When in Spain, wherever you are, steal away to San Sebastian and stay as long as you can. There are several flights in and out daily - Barcelona is only a couple hours away by plane and Bilbao is 1 hour on the bus, where you can then connect with many other flights if you need to get somewhere quick.
Beach: Rent a cabana for a few hours on Playa de la Concha. It’s the equivalent of $12USD for all day but you will feel like a million bucks.
Food: Pintxos, pintxos everywhere. Go to the Parte Vieja (the Old Quarter) and be shameless. Have a different pintxo at every bar along several streets. Under no circumstances should you roost at just one pintxo spot. Go hog wild - this is the town for it!
Spa: La Perla is housed in a portion of the old beach boardwalk structure and has a fresh water pool in addition to every spa service and amenity imaginable. Just book it. I hear it’s best to book as soon as they open or the day before.
Hotel: San Sebastian has loads of hotels but you need to book early or, if booking last minute, be flexible about style of accommodation. It’s all about the beach, so only settle for a hotel near the river if there is no other alternative - it’s not that far away and it is completely safe but spoil yourself in this town.