The Lisbon Oceanarium is excellent value for money and a perfect rainy-day pick if you want to see impressive marine life. It’s one of Europe’s largest indoor aquariums, with a huge central tank featuring sharks, rays, sunfish, and tuna, plus diverse habitat zones. Plan 2–3 hours; adult tickets are around €25 but can be discounted depending on promos. Nearby, you can ride the cable car and explore Parque das Nações