The Reserva Natural "Riberas de Castronuño-Vega del Duero" is full of beautiful routes to walk. You can ask me for them. It will a pleasure to explain to you everything.
You can make the routes marked along the Sanctuary. The main of them is the called the "Senda de los Almendros". I also recommend you to go to the Casa del Parque and visit Cristina and Alberto who will say to you the beginning of the route, too.