Evidently our unusual spell of nice weather (which didn't melt all the late September snow at high elevations) is gone--snow down to 4,000 feet today, and possible freezing weather at sea level Mnday night/Tuesday morning. More rain is forecast Halloween and onwards, so we are evidently going back to a normal weather pattern--i.e., wet.
I strongly recommend William Sullivan's guidebooks, particularly the Central Cascades and Coast Range/Coast volumes. While oriented to day hikers, the books suggest are lots of backpacking options, and you can often put several trips together. It also lists interesting things to see on the way to/from the hike.
The McKenzie River Trail is relatively low elevation and has hot springs along the way. There are trails along the Santiam River but not long enough for a backpacking trip.
There are some other Corvallis folks on the portlandhikers.org forum, which despite its name represents a statewide organization (Trail Keepers of Oregon).