El Parque Natural del Garraf is one of the most naturally beautiful places in Barcelona in my opinion. It has stunning views from it's various viewpoints which are equipped with picnic tables and tarmacked areas for picnics.
I would recommend that you wear comfortable shoes if you're to visit as it is in the mountains, and take insect repellent as well. Especially if you go in the summer!
I visited this park with my boyfriend. We had an afternoon picnic with the whole of Barcelona at our feet and it was a truelly wonderful experience. After we walked a little, before heading back with a camera full of stunning panoramic photos.
I'd definitely recommend it!
(You'd need access to a car!)