SFO is closeby, worth the trip to see the sights and get some good food and go to some clubs if thats your thing.
Castro is nice in Mountain View, some nice restaurants to eat at. If you want to stay in the city and not drive much, there really isnt much else to do during the day. But a short 1 hour drive away, theres a lot of stuff to do.
You can check out Big Basin Redwoods forest, its about 45 minutes from there, massive redwoods and one that you drive through, and another thats completely hollow in the middle, quite fascinating if you are into that sort of thing.
Else get to the coast line, Half moon bay, Stinsons Beach on the North, Monterrey, Big Sur on the South. All lovely places, to visit, easy to plan a day trip since its just a short 1-2 hr drive. Monterrey has a wonderful aquarium that I've visited several times, there are some good restaurants and beaches in that area as well. Ofcourse theres the world famous Pebble Beach right next to it, with 17 mile drive, which is a tad overrated IMO, but a nice place to visit nevertheless.
Then theres the Tech museum en route to SFO, if you are into tech stuff, its slightly kid oriented, but theres still enough stuff there to see for adults. Its cheap, but they close early so that takes up the day, which I dont quite like that much.
There are a few massive malls in the area. If you are into shopping, Valley Fair is the slightly upscale one compared to Great Mall which is huge and is filled with outlet stores so you can get some good deals there. So choose your poison.
Theres Shoreline park in Mountain view, which is a nice relaxing lake and park with some birds and animals hanging out with the people. Its very nice for a calm relaxed evening walk, it may appear crowded when u enter, but its easy to get away from the crowds since there are trails all over the place.
If you are like me and like Nature, and love the coast, I highly recommend Point Lobos State Park over Monterrey or 17 Mile drive. It is heaven on earth as someone once said, absolutely gorgeous place you can spend all day walking around. But do check the weather and visibility before going there, if its completely foggy, not much u can see or do.
EDIT: Oh yeah how could I forget the Mountain view art n wine festival, I'll most likely be there tomorrow as well.