Check the BART (Bay Area Rapid Transit) schedule -- you can take the train from SFO to downtown (I think it's about $5), or a cab ride is about $30 with no traffic. The restaurant in the FS at 757 Market is excellent -- floor to ceiling windows looking out to the street. There is a also a large mall with Nordstrom and other shops directly above the BART station. If the weather is ok, a short walk from the Union Square station is Maiden Lane -- there is a deli/restaurant there that blocks off the street, sets up tables outside, and sometimes plays live music during lunch time. You can also catch a cable car at Union Square if you're looking for the touristy thing. There is also a Cheesecake Factory on top of the Macy's -- the view is incredible if it's a clear day and you can eat outside.

Have fun!