Hi, I'm Ferdi from Munich, Germany. I arrived in Santander in September. It’s a really beautiful city, and because of the late summer, we still have a chance to play a lot of outdoor sports. 
The ESN Erasmus group created some WhatsApp groups for each sport; however, sadly, they are basically inactive. Only the football group met once every other week. 

But once you have your 4 or 5 people to play, it’s all good. Paddle tennis is pretty big and since it’s beginner, friendly and you have a central location, Duin Gym, it’s pretty good and easy to play. They also borrow rackets.

Tennis is not that well-established. There are three locations where you can play, but one of them (Beup Gym) requires nonmembers to reserve in person.  The other one is in  Montverde and has a really beautiful location. Next to it are horses, sheep, and so on. They also borrow rackets. The third one is the city's sports complex, Ruth Beitia. It has amazing facilities offering all kinds of sports. They don’t borrow any rackets. 

Luckily, the university itself also organised some competitions, which were a lot of fun. I recommend joining them; however, sometimes you have to find out about them through Instagram or so. 

Btw don’t even think of trying to play in Parque Magdalena. I tried, but it’s a social club and membership fees are somehow in the 10s of thousands 😅