MT. bike

The road into Pamplin Grove is paved, but the pavement stops at the West end of the meadow where it circles back behind the meadow where the group area and tent area are.
There are other places to ride as well... Cheetam Grove, Swimmers Delight and others, depending how far you want to go to ride.
I can't say I'd recommend riding on Highway 36, just don't know your riding ability... I've cycled 36 from 101 to Red Bluff and it's a tight road.
Little or no shoulder in a lot of places and occasional logging trucks...
Pamplin Grove is very close to the famed Tour of the Unknown Coast century race course, touted as California's Toughest Century... A natural 100 mile loop that starts and ends in Ferndale.
Areas of that course include the Avenue of the Giants, Bull Creek which BTW is my favorite redwood tree area, the Mattole Valley, Honeydew, Petrolia, an 8 mile stretch of rugged coastline and the Wildcat, featuring the Endless Hill coming up out of Bear River at Capetown.
If you do that course on a mountain bike, give yourself about 12 hours because of lower gearing/rolling resistance.... About 6 hours on a road bike. (If you're in good shape)