Technically in Mountain View rather than Sunnyvale, the Computer History Museum is the indispensable Silicon Valley museum and a short drive west of central Sunnyvale. The main exhibition, "Revolution", traces 2,000 years of computing in twenty galleries, from the abacus through the Babbage Engine to the present.
Highlights include an operational Difference Engine No. 2 built to Charles Babbage's original 1849 designs, a working PDP-1, an Apple I prototype, and the original Google search server rack. Allow at least two hours; the museum closed Mondays and Tuesdays. Admission a few dollars and free for under-12s.