Might be a little late on my reply, but we just did this drive in reverse - started in Klamath Falls and drove to SF. We went southwest on US 97 to I-5 briefly where we exited onto Highway 89. This basically takes you about 3/4ths of the way around Mt. Shasta - very pretty. Then south on 89 to Lassen Volcanic National Park - very unpopular yet pretty park with some beautiful high altitude lakes and a pretty drive that takes you about 1000 feet from the summit of the peak there. Also some neat geothermal features in the park. Then it was back West on Highway 36 to Red Bluff and I-5/505 to SF. Very easy drive to do in a day - if you don't spend too much time at the park. We also hit the Jelly Belly factory in Fairfield about an hour out of SF -- if you have kids (and even without to be honest), a fun thing to do!

Enjoy!

-Mike