We went whale watching in Mexico earlier this year, so I highly recommend a trip with the kids. Groupon is offering $35 tickets today (normally $70), and they don't expire til 4/29/10. The tickets are only good Monday-Friday, so that may be a hitch for you. But they guarantee you'll see a whale on this five hour trip, or the next trip is FREE.
You'll ride on a catamaran, and you can bring your own snacks and wine/beer. In addition to looking for whales, you'll get a sight-seeing tour of Fort Cronkite, Muir Beach, Stinson Beach, Bolinas and the Golden Gate Bridge. The boat also has an enclosed heated cabin (as well as a sun deck).
As for the whales, you should spot them in the Gulf of the Farallones National Marine Sanctuary, with a bonus of spotting elephant seals, dolphins, and porpoises too.
Tours begin December 1st, and you can buy up to six tickets with this deal.
For more info on the Groupon $35 whale watching deal, click here.