I live in Sonoma county, just north of SF. I would advise to do google maps driving estimates. I still forget that LA is 8 or 9 hours drive south on a good day.
Lassen is good but there is not much between SF and it, but a bit further north Lava bed national park is supposed to be awesome.
Yosemite will be baking and packed in August. It is very pretty though, be aware of bears and the crowds. We drove around it mostly as we were coming in through the Nevada side but took us an hour or more to park in the valley.
I loved the redwoods in the north of the state - the avenue of the Giants especially.
If you take the 101 north of SF stop off at a few wineries as most have something else to interest those who can't drink - fantastic views, wine cave tours, croquet. Francis ford Coppola has a winery, sells on the name mostly but has a good collection of film memorabilia.
If they liked Charlie Brown then take them to Snoopy's home ice, a skating rink in Santa Rosa that Charles Shultz had built for the town and has a snoopy museum. He lived in either sabastopol or Petaluma.
If they are film buffs look up the different filming locations from LA up to Mendocino county - something for every genre. Hitchcock fans have lots to see and some of the 90s horror films were filmed here too such as scream and ikwydls.
The 1 - coastal road - is picturesque but has very sharp turns and cliffs. It Is better to do from SF up than the other way.
The 5 is more inland but I prefer it for the drive back down from Lassen to SF. Still picturesque but no 100 foot drops into the ocean.
The jelly belly bean factory is between SF and Sacramento, go on a weekday to see the factory in action during the tour. it is free and you get to try any of their flavors.
I have only done the historic old part of Sacramento with the train museum, we went as DH went as a child before moving to the UK. I would only suggest if they are into trains, DF and DH loved it but DM and I were a bit bored.
Not CA but if you are going through Yosemite then Vegas might be an option? hotels there are surprisingly under 21 friendly. You could drive up from LA and go through Yosemite the other way.
DH lived in LA for a while, Pasadena. We did mostly touristy things there - Disneyland, universal and the open top bus tour. DH wouldn't let me not mention his favorite burger joint, 50s decor and they have been running it as long I think. Real mom and pop business - 'pie and burger' in Pasadena.