At 10 p.m. at the Sala Montjuic open-air cinema they always show a short film made by a Catalan studio, which means you have 15 minutes to return and rent a longue chair. Though the screen is rather small, it can be seen fairly well from any point. And you can always find room for two in the front rows. Check out the website for the timetable and prices. It's also recommended to buy tickets online in advance.