I second Highway 138 (I grew up in Roseburg). Check the roadside viewpoints, too; there are some spectacular rock formations visible from some of them.
If going down to Roseburg (to 138), take the MacKenzie Pass highway (if open, I forget the number, Sisters locals can tell you how to get to it). Large lava fields, with narrow road cuts (through the lava rock), scenic views of the Three Sisters mountains. It can also be done out and back in about a half day from Sisters.
138 also leads to the north entrance to Crater Lake NP, either via US97 to the east or I-5 to Roseburg to 138 in the west. But call the park first to see if the north entrance is even open. Most of the year it is closed due to deep snow. The south entrance is much, much farther from Sisters. If you do get to Crater Lake, there is a 33 mile road around the caldera, with many different views, worth taking (if open).
Very photogenic, not far east of there, is Smith Rock State Park, near Redmond. Hiking trails all over it, usually many rock climbers from all over the world.
If you like bridges, this one is not far from Sisters: <
https://en.wikipedia.org/wiki/Crooked_River_Railroad_Bridge>. If you go to that one, you might as well catch this one, too: <
https://en.wikipedia.org/wiki/Crooked_River_High_Bridge>. Just about anywhere on the Crooked River looks like the gorges shown in the bridge links.
If you like lava fields, but MacKenzie Pass is closed, just south of Bend is the Newberry National Volcanic Monument <
https://en.wikipedia.org/wiki/Newberry_National_Volcanic_Monument>.
Have a great trip!