Klamath Falls area?
You could camp at Mt. Shasta on the way up and then Mt. Lassen on the way back.I figure you are planning to have the trip sag-wagon supported so the route doesn't have to be contiguous.
There are lots of very good rides.
#1 - The Oregon, California & Eastern rail trail. Great scenery. Starts out paved - then hard-pack.
#2 - Sprague River Road from Chiloquin to Beatty - light traffic - little towns with corner stores.
#3 - Lava Beds National Monument - light traffic, hillier, volcanic landscape
#4 - Highway 140 from Lakeview to Adel - very light traffic - killer downhill usually with tailwind
(Hwy 140 from Beatty to Lakeview doesn't have much traffic, but enough and is narrow)
#5 - County Road 310 from Adel to Plush in the Warner Valley - great wildlife and lush riparian zone
(steep dirt road to Hart Mountain - but there are hot srpings there and camping)
#6 - Surprise Valley from Fort Bidwell to Eagleville - practically no traffic, little stores along the way
(plus they could do just a bit of riding in Nevada and make it THREE states!)
#? - Crater Lake Road - much more difficult, narrow, still may have snow - probably not
It's 75 miles from SLO to Kettleman City; then 430 miles to Mount Shasta on I-5.
If you left early enough, there would be time to play in the evening at the campground.
Reservations highly suggested for Mt. Shasta and Mt. Lassen in the summer.