Today, I took the trip from Halsey to Sisters via OR-228 E (Sweet Home) and then US-20. After having passed Sweet Home, a sign tells you "NEXT GAS IN 70 MILES", that's true, no kidding.
Remember that in winter-time (extra canisters!), rather gas up in Sweet Home - relatively cheap there, today $3.559 to $3.599 pg (regular).
You have to bear in mind that you drive mostly uphill and sometimes downhill on US-20 East. Passing Cascadia, you enter Willamette National Forest. On your left hand side, you might see the mountain formation "3 Fingered Jack" (7'841 ft) but surely you see Mt. Washington (7'794 ft) at several viewpoints on your right.
I gassed up in Sisters at Chevron's for $3.599 pg (regular), neat "western-style town".
Pass Sisters till Redmond, and you see the formations "The Sisters" (10'085 ft N, 10'358 S) - Mt. Washington and Mt. Jefferson (10'497 ft).
My return trip was via McKenzie Pass on OR-242 W to Springfield OR- 126 W and then I-5 N to Exit 216 Halsey / Brownsville and Sweet Home.
The Hwy OR- 242 is quite challenging, you have quite a lot of slow-pokes ahead of you. No wonder that this road is prohibited for RVs and other vehicles longer than 32 ft. You really face hair-needle curves there - RVs with trailers cannot manage this, for sure.
The McKenzie Pass on elevation 5'325 ft has an awesome viewpoint. The Dee Wright Observatory in the middle of a lava "moonscape" offers you a unique view to the mountains around you.
This emperor's weather today provided 83 °F at this viewpoint, more than 5'000 ft above sea level, can you imagine this?
En route to Eugene/ Springfield on OR-126 W, you have the McKenzie River on your left, almost all the time.
Begin of excursion: 9:00 am
End of excursion: 4:15 pm