There is so much to see and do that we would suggest setting aside at least 2 days if you really want to experience everything the center has to offer.
Of the 6 buildings of the City of arts and sciences, 4 are open to the public: The Oceanografic, the Principe Felipe Science Museum, L’Hemisfèric, and the Umbracle.
The Oceanografic is an open-air aquarium and the largest one in Europe at that. The 110 thousand square foot structure holds over 40 million liters of water and is home to 500 different aquatic species including marine mammals (dolphins, beluga whales, walruses, etc.), wetland bird species, aquatic reptiles and of course an immense variety of fish, crustaceans and other sea creatures.
Due to the immense size of the center, you will need the whole day to see everything.
The Principe Felipe Science Museum is a 40,000 square meter interactive science museum. You will find all types of interactive exhibits spanning everything from the human genome to space travel. Especially if you are visiting with children, you will need at least 4 hours to get the full experience.
Then there is the Hemisfèric and the Umbracle. The Hemisfèric features an IMAX cinema, a planetarium, and a laser show display. The Umbracle is an open-air structure that houses indigenous Valencian botanical species, and a collection of sculptures and contemporary art. You will need about two hours to visit both centers properly.
The Umbracle also features a brand-new disco, so you might have to set aside a good chunk of your evening as well if you feel like unwinding with some music and a cold drink.