If you wanted an alternate route down to Bend, I'm very partial to US-197, which I-84 and US-30 meet at the east end of The Dalles (which also has a very charming downtown). OR-35 is a very nice drive and has the advantage of being right next to Mt. Hood. However, you'd see more of the Gorge, and US-197's a fun road and has its own rather nice scenery (miles of wildly undulating wheat fields out of The Dalles, and some desert canyons, particularly winding in and out of Maupin, plus you can still see Mt. Hood), the elevation is several hundred feet lower, and it won't have as much traffic (particularly if ski season's gotten going). 197 ends at 97 about 40 miles north of Madras. If you don't check it out this trip (I can understand wanting to go up on Mt. Hood), I'd definitely keep it in mind if your travels take you back to Oregon. I'd absolutely recommend against taking 26 directly to 97. Between Portland and Mt. Hood (particularly around Gresham), the traffic can often be aggravating and it's not particularly scenic until you get a good bit past Sandy.
Bend's among my favorite places in the whole state (and if I could figure out a way to make a living there, I'd move there in a heartbeat)--there's definitely a lot to do there, and I'd certainly recommend spending a night there. The downtown is fantastic (Drake Park should be absolutely gorgeous right now), and there's a lot of great restaurants around downtown, the Old Mill District (there's a great "authentic" pizza place there--Flatbread), and several breweries around besides Deschutes (Cascade Lakes Brewery/Pub on Century Drive is usually a stop for me--food is great there).
As far as Sisters goes, I'm honestly not a huge fan of the place. It used to be pretty quaint in the early/mid-1990s, but it's become a bit of a tourist trap and it can often be rather crowded there (even now). The only big reason I'd go is if you wanted to take the Old McKenzie Pass Highway (OR-242), though unfortunately,
it just closed today (the shortest open season they've had for it ever--it only opened up July 15th this year).
Cascades Lakes Byway (also in part known as Century Drive because it's 100 miles long--same road Cascade Lakes Brewery is on, actually) also closes down in the winter beyond Mt. Bachelor, though it's a more modern road at a lower elevation than McKenzie Pass, and so it's usually open most of the year (and should be open now, though I'd check TripCheck to be sure). Absolutely stunning drive--some of the lakes are right next to the highway and the color of the water is usually a rather interesting shade. OR-58's great, too, and you'd definitely see some fall color up there, though Willamette Pass is rather high (though being a major route, should be well-maintained). There's also a couple historic covered bridges along it (including the one in Lowell at Dexter Lake).
Oh, and in Redmond, there's an amusingly bizarre gas station on the south side of downtown (right at the southern terminus of US-97 BUS on 5th St, near where it meets OR-126--might be able to see the sign from the new US-97 Redmond Bypass). Has some of the cheapest gas in the state--just look for the sign with the smirking, crazy-eyed anthropomorphized plum (I'm not making this up, I swear

).