If you want to take a loop around to the crater then:
Head out of Mt Shasta east on HWY 89 and then Co Hwy A19 over to Hwy 299 @ McArthur. After that head NE on 299 to 139 up to the road that leads into the South entrance of Lava Beds. Ride through Lava Beds and then up to Klamath Falls and continue North to Crater Lake.
This should be a good days ride with some time for Lava Beds.