Not that it is a big roller coaster but watching the whole city of Barcelona while being up there, taking a scary look at the valley at your feet and raising your hands to enhance the thrilling experience is definitely a different way of approaching our city.
Take some time to visit Tibidabo. If you have kids it is really a must-see. Barcelona Photoblog has published many pictures about this park but I will only suggest a very similar shot: Amusement Parks - Roller Coaster Panic.