Who cares if it's a Barcelona cliché. Give in to the urge and get to Plaça Espanya to see the Magic Fountain show. It's always crowded, but everyone should see it at least once, and it's free.
The show is especially gorgeous during the closing ceremony fireworks for Barcelona's city festival, La Mercé.
Prefer a view from above? Head to Las Arenas, a shopping center in Plaça Espanya housed in a renovated bullfighting ring where you can watch the show from the rooftop.
Nearest metro stop: Espanya (L1 and L3)