Six Flags Discovery Kingdom, on the northern edge of Vallejo, is one of the few theme parks in the world that combines roller coasters with live animals and marine shows. It is the main single-day visitor draw in the city, pulling families from across the San Francisco Bay Area.
The park has a dozen major coasters including Medusa, one of the first floorless coasters in the world when it opened in 1999. Animal exhibits include tigers, lions, penguins, and various marine mammals. Dolphin shows and killer whale presentations (the orcas have been phased out from many parks, but Discovery Kingdom's position continues to evolve — check current programming) run through the day.
The park is on the northern edge of the city, easily reached from I-80. It is a full-day proposition; buy tickets online in advance for significant savings over the gate price. The park is open year-round but with reduced hours outside summer.