Good suggestions so far. However, traffic in Portland to get onto US 26 can be hectic, and Hwys 26 and 97 are both 2 lane most of the way, and can be slow going depending on traffic.
I would suggest I-5 south to Roseburg, OR, then Hwy 138 east to Crater Lake, Hwy 138 follows the North Umpqua River part of the way and is quite scenic but not a high speed road in places. Traffic is not a problem, the grades are much easier and there are no mountain passes to cross. And, it leads right to the north entrance to the Park, where the other route would take you.
Crater Lake is one of my all time favorite places, spent a lot of time there over the years. Enjoy your trip!
