Allow me to add my $0.02 worth. I have lived up and down the west coast from San Diego to Everett, WA. I'm making my home just south of Tacoma these days, but I've driven 101, 1, and 5 for over 30 years in all weather. I5 is for if you're in a hurry and need to get somewhere. The coast roads are for the drive. When tourist season hits the coast it's wall to wall RV's, and traffic sucks. Prices go up, and everything gets crowded.
Apparently this isn't clear to most folks; driving I5 and driving the coast roads are completely different! It takes most of a day of hard driving to get from one end of Oregon to the other. The coast road is mostly two lanes (one in each direction) and goes thru dozens of small communities with associated traffic and stops. Likewise, the California coast; it's a good days drive from Santa Cruz to Santa Barbara, and having driven it dozens of times I still stop to just sit and look at something on the way. South of SB you hit traffic, and unless you detour to the Freeway (I5 again) you're going to be in stop and go all the way to Mexico.
I'd fly into SeaTac and drive the route Olympia/Aberdeen/Long Beach/Astoria to get on 101. Seriously the Lewis and Clark Museum at Cape Disappointment is worth stopping for. Especially if it's not raining. Likewise I think driving over the bridge to Astoria is something everyone should experience, but I like bridges.
Take a couple weeks and do it slow. Take a hike in the rain. Realize there are people (like me) who go out to the coast just to sit on the beach and watch the storm. (Or better yet, in a condo just off the beach with a view.) Hiking the forest in the rain in nice too.
If you start in Seattle you have two options; go out and around the Olympic Peninsula or not. If you do decide to go the long way you should drive down to Olympia and then up to Port Angeles, from there to Forks, and then down the coast to Aberdeen/Grey's Harbor. You can easy make that loop in a day, but I usually stay over in Forks or out on the coast at La Push. You should be able to get a beach condo for $40-$50/night.
From Aberdeen you go along the coast to Long Beach. Most of that side of the coast was logged in the 60-70's and still has some going on. (They work in the rain.) There's a really good museum for Lewis and Clark in Long Beach. Then you have the bridge to Astoria. If there's weather that can be a ride. The bridge does move! That's one of the most dangerous river's in the world at that point. But Astoria is nice, an old fishing village. Just south is Tillamook, which is famous for cheese and ice cream. They have one of the best collections of old airplanes you can ever see in an old blimp hanger just outside town.
From Tillamook you come a little inland, but you can see signs to take you out to some of the lighthouses of you want. From Lincoln City to Newport is real pretty. And if the sun comes out you can probably see some seals or otters on the rocks. That stretch is famous for crab and lobster so make sure you eat local.
I love the drive from Newport to Coos Bay. You pass the best sand dunes north of Pismo Beach in that stretch. Coos Bay is a nice town to spend some time in too. But from there down to Klamath (in CA) is the most rural stretch of road you'll hit. It's the sticks, and services are few even in summer. Plan accordingly.
There are several good lighthouses you may want to stop at. Pt Arena in CA and Yaquina Head light in OR are among the best. Plan a walk at Pt Reyes too! The Sea Lion Caves are a tourist trap. If you want to see them for free go to Yaquina Head and look at the rocks at the base of the cliff. If the tide is out you can walk out a long way and there's all kinds of tide pools.
More to follow...