Hospital de Sant Pau

• Underground galleries that once moved patients between pavilions, now open tunnels kids can walk through
• Free for under 12s, with self-guided routes you can take at a child's pace in about 90 minutes
• Shaded gardens and tiled interiors that stay cool, a good midday stop when the city bakes

Designed by Lluis Domenech i Montaner and built between 1902 and 1930, this UNESCO-listed site is the world's largest Art Nouveau complex: 12 restored pavilions covered in colored tile, sculpture, and stained glass, set among open gardens and linked by long underground tunnels kids can walk through. Self-guided routes pass mosaic murals, domed ceilings, and a scale model of the campus.