Public buses are available at reasonable prices. Each journey costs 1 euro (more for journeys out of the city proper, or you can save 50% by buying a 10-journey 'Bonobus' at a 'Tabaco' shop). Specifically, buses can take you to the main beaches some 2-4km (2m) from the city center.
Taxis are widely available throughout the city.
The City offers free bicycle rental at a number of locations around Santander (Jardines de Pereda, El Sardinero, La Magdalena). Bikes are loaned for the day, so get in early to avoid missing out