July through October is prime whale watching season here. It's when the Humpbacks and Blue Whales spend time fattening up on krill off of the coast of Northern California before heading back down to Baja to breed. A trip to the Farallon Islands, 27 miles west of San Francisco, can be one of the best ways to get up close to these giants of the sea.
You can get out to the Farallones on tours organized by SF Bay Whale Watching, which depart early in the morning from Fort Mason (weather permitting). The two hour haul to the Farallons is a bit harrowing, which is to say strong stomachs, sea legs (and potentially Dramamine) are required.