See Gaudí's masterpiece from outside or enter to view the interior; interiors usually open ~9:00–18:00 but times vary — book tickets in advance if entering.
Take the R5 train from Plaça Espanya and cable/funicular up to the monastery for dramatic cliffs and hiking; services generally run from early morning to evening — check schedules.
Admire the tower from the outside and take photos from Trocadéro for free; climbing the tower has a fee, so viewing from below is best for shoestring travelers.