To see Redwoods head up highway 101 towards Redwood National Park. On the way make sure to take
Avenue of the Giants. It's a 30 mile scenic road through a redwood forest that parallels 101. You should see signs on 101 around Garberville. The redwoods in northern California are coastal Redwoods as opposed to the Giant Sequioas found in the central Sierras. The coastal redwoods aren't nearly as wide as the Sequioas but I believe they're the tallest trees in the world.
After you come back from Avenue of the Giants to 101, check out the town of Ferndale. You'll be coming down a mountain out of a dense forest and suddenly there's this little town that has about the cutest houses and downtown in California. Much of the buildings date back to the 1800s, and it's a great place to find a bed and breakfast for the night.
As for Oregon, the coast is indeed nice but also consider heading inland to Crater Lake if you have time. In March there will still be snow but it's definitely drivable and quite beautiful.