It is possible to take the train down to Palo Alto from SF, rent a bike from the bike share system, take a leisurely ride down Palm Drive and around Stanford campus (2-5 miles along bike paths) and then back to Palo Alto for coffee (or something else tasty). Coupa Cafe is a good place for coffee, as ulfw mentioned.

If you want to tour one of the big companies, find a friend/acquaintance/recruiter, and catch lunch. These companies compete on quality of meals...

Also, if you're going to rent a car, I highly recommend visiting some Redwoods. The best around are in Big Basin about an hour south of SV. There are also some just north of SF in Muir Woods.