I've never been to California so it's a window on another world for me.
And to think it's just a single overnight away by Amtrak. You ought to come on down! It's a beautiful trip.
Here's your Itinerary... four days by Rail, and possible overnight in Seattle before and after the Trip.
Day 1: Depart Seattle 9:45 AM on the Coast Starlight. See Puget Sound, the Columbia River, the Willamette Valley, and climb the Cascades (by daylight if it's Summer and the Train is not too late).
Day 2: Northern California, Sacramento, the Bay Area, the Salinas Valley. San Luis Obispo (with Horseshoe Curve and Stenner Creek Trestle). Magnificent ocean vistas from Vandenberg AFB through Ventura (if the Train is not too late... or if it's Full Moon!). Arrive in Los Angeles 9:00 PM (theoretically), transfer to San Diego on the 10:10 PM Train (or a Bus if later). Arrive San Diego about 1:00 AM. Catch a few hours of sleep at a nearby Hotel.
Day 3: Catch the 6:10 AM Surfliner northbound to Los Angeles, with beautiful morning views of the Pacific Ocean and Beaches. Transfer to the 10:15 AM Coast Starlight. Enjoy the Coastal Views through San Luis Obispo by daylight, and the Bay Area late evening.
Day 4: Be awake early to see Mount Shasta at Daybreak. Continue through the Cascades, then familiar territory back to Seattle at 8:45 PM (theoretically).
By spending two nights in San Diego, you would have an entire day free to enjoy that pleasant City, and ride the Trolleys and have a nice Mexican Dinner at Old Town.
Unfortunately, Amtrak provides no same-day connections for the Starlight to points in Washington State north of Seattle, in either direction. So this leg would have to be either on your own, or on the Cascades Trains with overnight in Seattle.
Coach Fares begin at $101 each way... $90.9 for AAA and $85.85 for Seniors (62+).
Just a thought...