In Humboldt Redwoods State Park there are Quite a few places that would suit your criteria of a Class B and boondocking.
As has been mentioned Albee Creek or Hidden Springs.
I like
Burlington Campground It is set in the Redwoods and has a visitor center next to it that has a gift shop and some really good exhibits in it from the early days of logging including a old truck from the era.
Sites 1 thru 18 and 53 thru 57 are walkup the rest are reservable but mid week it is mostly empty. There is hiking and a trail to the river.
In my
Profile you can see what it looks like.
Good luck and have FUN..