Hi Wendy,

The other caves with prehistoric art in the area that I'm familiar with are the caves of Tito Bustillo. Here you visit the real caves, not a reproduction. They are open Wed- Sunday and the first visit is at 10:25, last visit at 4:15PM. Tickets are timed and each group can stay for 60 mins. The only issue is that they are outside the coastal town of Ribadesella( beautiful beach town by the way) and that's approx. an hour and a half from Santander. I don't know if you have enough time to do it, even if you get a private guide to drive you there.

With respect to Santiago, in addition to train service there is frequent bus service between Vigo/Santiago, Santiago/ Coruña. If you want I could try to find the schedules for you. The bus system in Spain is excellent. I haven't done the Vigo/ Santiago trip but I have done the Santiago/ Coruña and back. You just have to make sure that you get in the express bus ( not the one that makes several stops or doesn't go through the toll highway)!

Editing to add: Santillana is indeed worthwhile to visit.