On a budget, weigh it — Oceanário de Lisboa isn't cheap for what it is.
~€25
Genuinely excellent — the giant central tank with sunfish, rays and sharks is mesmerising, and the design lets you orbit it from multiple levels. The catch: it's pricey, busy with school groups and families, and it's out at the eastern Expo grounds, a metro ride from the historic core. Far and away the city's best rainy-day and kids option.