There is a picnic area called Parc Fluvial located in Vilanova del Cami, Barcelona. It offers amenities such as a beach bar, barbecue facilities, and more.
We had planned to spend the night there, but we had to leave because it was crowded and noisy. There were BBQs, a bar, and many people sitting in the park with loud speakers. Perhaps going on a weekday or during a different time of the year would be more peaceful.
The place is nice but there are two factors that lower its rating: young people during summer nights (causing noise) and a nearby bar with music. It is recommended to stay for only one night after consulting with the local police.