You'll also have a to pay a toll if you cross the Golden Gate Bridge. You cannot pay cash, no toll takers. You can pay in advance online or you will need a transponder with your rental, which can be expensive. We were told by Budget that we would have to pay $4 a day for the transponder for each day of our car rental, even though we'd only need it for the one time we'd be crossing the Golden Gate Bridge.

If you drive from Portland to the Oregon Coast, definitely see Haystack Rock (as others recommended), but you might prefer to stay in Manzanita, 18 miles to the south, as it's smaller, less touristy, and accommodations are cheaper. We stayed at Sunset Surf Motel, which was simple but clean, across from a beautiful beach, and had a microwave and refrigerator. As we were checking in, two bald eagles flew overhead. It was wonderful!

Ecola State Park, north of Cannon Beach, and Oswald West State Park, between Cannon Beach and Manzanita, have hiking trails.

When driving south from Manzanita, you'll see some signs for Cape Meares Lighthouse. It's part of the Three Capes Scenic drive (part of which is closed right now). If you have time, try to visit Cape Meares Lighthouse. It's a beautiful spot with many Sitka spruce trees.

Oregon has beautiful lighthouses. I loved Yaquina Heads. On the beach below Yaquina Heads Lighthouse, you can explore the tide pools at low tide. Rangers are standing by to tell you all about the tide pool creatures, such as purple urchins, green and aggregating anemones, sea stars (starfish), and small fish. We were there a couple of weeks ago and also saw numerous harbor seals nursing their pups.

Haceta Heads Lighthouse is another fabulous lighthouse in a stunning location. When we were there a couple of weeks ago, three grey whales were feeding in the waters right in front of the lighthouse. We saw whales all along the Oregon Coast.

Speaking of whales, you might want to have a look at this news about 200 whales settling in at Oregon's Depoe Bay for the summer:
http://koin.com/2016/05/31/depoe-bay...me-for-summer/

After you cross the border into California, you'll reach Crescent City. You can get brochures and great advice about the Redwood National and State Parks and its hiking trails and campsites from its Crescent City Office, 1111 Second Street, Crescent City, Calif. (Corner of 2nd and K Streets)
https://www.nps.gov/redw/planyourvis...CP_JUMP_281933

I highly recommend you hike through Prairie Creek Redwoods State Park:
http://www.parks.ca.gov/pages/415/fi...ochure2014.pdf
It's south of Crescent City, but north of the small community of Trinidad (which is north of the college town of Arcata).
http://redwoods.info/capsule.asp

Driving south of Eureka on Highway 101, you'll start to see signs for Avenue of the Giants.
http://avenueofthegiants.net/

You'll save time if you just drive straight down Highway 101 to San Francisco, but in my opinion, you'll be missing some of the most beautiful parts of northern California if you don't divert over to Highway One at Leggett, but it could add a day or two (or more) to your trip. The Mendocino County Coast is beautiful, followed by the beautiful Sonoma County Coast, then the Marin County Coast. But Highway One can be extremely curvy. The northern California Coast can also be foggy in summer.

If you don't have time to drive the entire length of Highway One north of San Francisco, try to at least do part of the Highway One drive from Point Reyes National Seashore to Muir Beach. You can cut over to the coast from Highway 101 at Petaluma or Novato. Point Reyes National Seashore has great hiking trails and camp sites (which you must reserve ahead of time).
https://www.nps.gov/pore/index.htm

When done at Point Reyes National Seashore, follow Highway One to Stinson Beach, then Muir Beach. Muir Woods Redwoods National Monument is near Muir Beach. It'll be crowded at this time of year. The park itself won't feel too crowded, but you'll have to park quite a distance from the park entrance.
https://www.nps.gov/muwo/index.htm

Highway One will join up with Highway 101 in Mill Valley, north of the Golden Gate Bridge. Before you cross the bridge, and if it's not foggy, you might want to make a right turn into the Marin Headlands to catch some fabulous views of the city and the Golden Gate Bridge.
https://www.nps.gov/goga/planyourvis...older-web2.pdf

Thanks everyone for your help! We had an amazing trip, it couldn't have been any better. We had amazing weather all throughout Vancouver, Washington & Oregon...strangely California was the only state that was pretty cloudy, and obviously some fog in the mountains.
All the ferry/tollbridge crossings went smooth, and gas stations were easy to find and use as well. I'm planning to write a proper breakdown of the trip, but roughly we went to the following places:
Seattle - Lake Crescent - Sol Duc Hot Springs - Forks - Lake Quinault - Willipa Bay - Ecola State Park - Canon Beach - Tillamook (cheese factory AND airplane museum) - Depoe Bay (saw whales for the first time in our lives!) - Yachats - Siuslaw National Forest - Heceta Head - Port Orford (can recommend staying in Wildspring Guest Habitat! our only expensive stay, the rest were cheap but awesome airbnbs) - Samual H Boardman viewpoints (Thomas Creek one was closed unfortuantly) - Arcata - Burlington - Avenue of Giants - Myers Flat & Legget (drive thru trees) - Russian Gulch - Mendocino - Point Arena - Point Reyes - Bolinas Lagoon - Stinson Beach - Golden Gate Bridge - San Fransisco, as well as other lakes and little towns I have missed off! Will be back soon for sure. Thanks again everyone for the tips and advice!

>>.strangely California was the only state that was pretty cloudy<<

Yep -- Summer is cool, even cold, on much of the CA coast. 100F+ degrees inland, cool and foggy along the coast. Autumn is when it s warm and sunny most days.

Glad you had a great trip. Looking forward to the trip report.