We stayed at Camping Don Quixote on our way back up to Santander last August, which is about 10-15 mins drive outside of the centre next to the river.
It's a nice, clean site with a pool and restaurant and good facilities.
Took a taxi into the city to explore in the evening and it's stunning to say the least in the streets around the cathedral and near the university.
Lots of places to eat and drink near the Plaza Mayor.
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Our only problems came on the morning we left when a huge thunderstorm hit the area and we had to endure our van being pelted with 50p-sized hailstone!
Camping Don Quixote