After passing the small town of Fertilia, you can reach the cozy Punta Negra beach. On one side there's a 4 star hotel with private beach but on the other side the beach is open to everybody.
This beach is not so crowded, and not far from Alghero, so it can be a good target for a bicycle ride (7 km.).
It takes around 15 min. by car but parking lot is small, so better avoid it in the weekends.