Just rode up that way. Coast highway whenever possible (rt. 1, then 101) until about halfway through Oregon, where it starts getting more congested and less scenic. At that point, think about heading away from the coast into the Coast Range, where there are lots of twisty local highways, or Cascades, where there are volcanic features not seen many places. During May, remember that much in the Cascades will still be snowbound, so check ahead. Remember also that it's still rainy season along the coast, so that as you get further north you're likely to see more greenery, more rivers and streams, more big trees....but correspondingly more logging devastation and more miserable, gloomy, rainy weather. Although you could luck out for a week.
The Lost Coast is wonderful in any sort of reasonable weather. I never get tired of the redwoods, and if you're a fan of large trees the rainy side of the Olympics (e.g., Hoh River Valley in Olympic National Park) is amazing.
You don't say what sort of riding you're interested in, or what sort of bike you're riding. More specifics from you might elicit more specific suggestions; personally, I favor backroads and dirt whenever I've got the time, but you could also easily buzz the whole route from the Bay Area to Seattle in a couple of days along the dreaded I-5 corridor.
Hope that helps.