How about the Crater Lake Century? The website has not been updated from the 2017 ride yet, but it will be. https://www.craterlakecentury.com/

Great scenery. Make that fantastic scenery. Good people. Start in blink-and-you'll-miss-it Fort Klamath. The century goes all the way around the lake, the metric up to a view point. When I have done the ride, I have made my own hybrid more or less metric route that follows the century route to the rim of the volcano and along the western rim until have had enough and then turn around and head for the barn. Sometimes I have included the century ride's flatland start, sometimes I have headed straight for the volcano. I don't tell them what I am doing (easier to get forgiveness than permission), but my sense is they don't much care as I am always on a road that is one one of their official routes.

The ride also has some funky unusual rules, meaning you are set off in small groups every 15 minutes and only after getting a briefing/lecture on rules of the road, including being told to come to a full put-your-foot-down stop at the entrance to the Crater Lake National Park and don't go until acknowledged by a ranger in the entry kiosk. It isn't a big deal, and they do it as a condition of getting the use permit from the park. Like I said, not a big deal, but if you rebel against authority it might chap your backside. I think it's an insignificant price to pay for having a supported ride in this gorgeous spot.