I work in Folsom and have taken mine on the nearby S. Fork American several times. The stretch from Chili Bar to past Greenwood Creek (Hwy 49), although not wilderness, is a lot of fun in a packraft.
Someone recently posted either here or packrafting.org about a nice trip he did with his daughter in Redwood Nat. Park (I believe the one Dave is referring to). It didn't sound very long, though they did have a nice campsite. I have often stared at my map of the Klammath River region looking for possible hike/raft routes along 299, but like you said, it's not so clear. I thought of maybe using Trees of Heaven campground as a base for exploring.
cacreeks.com has a list of runs by difficulty with info about each.
As far as ultimate California packrafting adventures go, it's not NorCal but one can't help but admire Wyatt Roscoe's Headwaters of the Kern trip report from last year. I have hiked along that river a few times and it's really beautiful.
Headwaters of the Kern
Let's get together and do some packrafting some time, Ken!