Take 126 east out along the McKenzie River, turn south on US Forest Service road 19, which for a long while runs along a fork of the Willamette River, and then west on 58 back to Eugene.
Most of it is in the Willamette National Forest, and the run up the McKenzie River and down the US Forest Service roads are great - very lush Redwood forests and lots of water. There is a cool water fall just off the road south of the Cougar Dam (the biggest earthen dam I've ever seen), and about 48 miles into the forest road there was a little pull off down to a one lane bridge that crosses the Willamette at a particularly nice white water area. Nice place to get out and walk around for a bit.
The whole thing took about 4 hours, so tomorrow you have plenty of time. Take some water - it is about two hours between places to buy water on 126 to the next one on 58.