I have three days free in Spain and am confused between Valencia and San Sebastian. I would be going in end of August and its two 25 yr old women. I like beaches, nightlife, culture but no museums. Would we have time to do any other places nearby ? We would be only using public transport.

Any recommendations ?

They are quite different and some km away from each other. You say you have some free days so where is your last stop before the free days? I assume you will be using public transport to get to one of these 2 places too.

Mid-August (15th-21st?) in San Sebastian is Semana Grande. It's crowded and more expensive, but also a big party. For only 3 days you'd have plenty to do.