I wouldn't recommend the Guggenheim. It's a fantastic building but the exhibits are a bit "meh"
If you want the beach - Jump ion the metro to Plentzia. Lovely walk round the river to big beach in Gorliz.
There are other beaches in Las Arenas and Sopelana, If you get off the metro at Las Arenas, you can walk past/over the Puente Colgante to a small beach.
The weather isn't great at the moment btw.
You can walk along the coast from Bidezabal to Plentzia, the views are great (it's where I go riding all the time).
You can borrow my bike for a day if you like.
Other touristy things - The Funicular up to Artxanda. The casco viejo is nice for drinking.
San Sebastian is a lovely place, so if you have time it's worth a visit.
That's just a random thought dump. If you have any specific questions - fire away.