Trinity Wilderness Trip - http://www.george4wd.taskled.com/tn2021.html is the Trinity Forest area when I last was there late last year. There's quite a LOT of forestry roads to explore, some well worn, some rarely traveled.
Usal Beach 2009 - http://www.george4wd.taskled.com/usal2k9.html one of several trips to Usal beach when my kids were kids

. There's not too much to explore in terms of roads - Usal Rd basically runs up the coast and will take you to Shelter Cove, a small town with great fish & chips near the musuem/light house. Usal Rd from Usal beach to the T-junction further north often has signs saying it's not for through use - but not like there's anyone telling you to not drive it.
Downloading various topo maps of the areas is a good idea if you want to explore the Trinity forest. Easy to get yourself lost if you don't know how to read a map and do some planning.
Neither are 'day' trips and at minimum I'd take a w/end and camp out. Nice thing with the Trinity area is that you are up at 5000-6000' feet most of the time so it's cooler even in summer when places like Red Bluff are bumping into 100F temps. Do note that big fires went through the area, so a lot of the forest beauty is now charcoal - thought there is still a lot of beautiful areas up there.
Southern deserts are getting warmer quickly...
Look north of Clearlake and you'll find Lake Pillsbury and there's a decent amount of forestry roads to explore. If you are in the north bay, that's not too far to head and feasible to do as a long day or sleep out the Saturday night.
Hollister is an offroad park, so really just a place to go to test out your vehicle and decide where to spend more money for upgrades

It can be entertaining to watch folk do neat things or stupid things
cheers,
george.