California's Great America is the South Bay's major amusement park, with roller coasters, family rides, live entertainment, and a water-park component in season. It is technically in Santa Clara, but it functions as a San Jose-area day out for visitors based in Silicon Valley.
Check the operating calendar before going, because hours and open days vary sharply outside peak season. The park is easiest by car or VTA light rail, and it is a full-day choice rather than a quick sightseeing stop.