I remember going to the Valley in 2002 and expecting tours of all the famous companies etc... I made the "pilgrimage" to Apple headquarters, expecting some sort of museum or something. Security basically escorted me out of the lobby. Intel does have a little thing you can view at least.
For the most part Silicon Valley is very uninteresting to just visit unless you like seeing corporate office parks from the outside.
That's kind of part of it though. It's a living, working institution. The best work is going on now. It's not a museum that people come and visit for historical purposes. Normally when something becomes a tourist attraction it's well past being a living entity. Intel and Apple are historic, but they're also well still alive. That's part of the energy of Silicon Valley.
But you can visit some of Apple's former buildings for free. Look on folklore.org for addresses. There's at least one 70s-tastic office park at 20863 Stevens Creek.
For the most part Silicon Valley is very uninteresting to just visit unless you like seeing corporate office parks from the outside.