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Photo: Museo de Altamira y D. Rodríguez, CC BY-SA 3.0 via Wikimedia Commons
The caves of Altamira, discovered in 1868, have prehistoric paintings as old as 36,000 years - they were added to or amended over another 20,000 years. The condensation from 20th century tourists damaged the paintings so the caves were closed to the public and a replica cave built - this is what you tour, along with the adjacent museum. The true cave is 500 m north but closed off. The tickets can be bought via the official page or on the spot - but in the latter case, expect you may be waiting 3h or more get in.
The Cave of Altamira is a cave complex, located near the historic town of Santillana del Mar in Cantabria, Spain. It is renowned for prehistoric cave art featuring charcoal drawings and polychrome paintings of contemporary local fauna and human hands. The earliest paintings were applied during the Upper Paleolithic, around 36,000 years ago. The site was discovered in 1868 by Modesto Cubillas and subsequently studied by Marcelino Sanz de Sautuola.
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Cistercian Monastery / abbey built in neogothic style 1906/10 and now run by their branch order the Trappists. You can attend mass and buy their famous cheeses.
Monte Castillo is riddled with caves containing Paleolithic art about the same age as Altamira. Two caves are open to view, and here you see the originals not a replica. But that means tightly restricted numbers, so you need to book ahead, you're unlikely to find availability on the day. It's worth seeing both: Castillo, and Las Monedas named for coins found there.