One great option for traveling down to San Francisco from Seattle is to drive along the coast. Just get on highway 101 and drive it straight down. There are a lot of beaches to stop at along the way. In Oregon, you can take the Three Cape Scenic Drive.
You can also go by train. The train starts as far north as Vancouver, BC Canada and goes all the way down to Los Angeles. Here is the link for the train. I've taken it from Seattle to Portland before and it's a nice ride. But with more than one person, it's definitely cheaper to drive.
Speaking of which, Vancouver is a nice city to visit. It's a only a couple hours drive north of Seattle. There is quite a bit to do there and the weather is almost exactly the same as Seattle. For us the draw is that just south of Vancouver is Richmond which has a lot of Chinese immigrants. There are a lot of great Chinese food places there.