I've only mostly visited the coast and Portland...the drive along the coast heading West from Eugene and all the lovely little stops along the way are a great way to spend the day. Little B&B's to stay at, beaches to explore - historical lighthouse at Yaquina Head was a wonderful visit! They have tours running during the day. Amazing little seafood shacks on the coast! We once took the 6 back inland to Portland, and the drive through the Tillamook State Forest was another joy. Or, you can go back more quickly on the 18 and hit up the many wineries around Willamette Valley if that's something you enjoy. The Evergreen Aviation and Space Museum (just east of there) is *lots* of fun.
Another trip we took was south of Portland to Silver Falls State Park - it's gorgeous! And on the way is the Gordon House (a Frank Lloyd Wright house) which is on the grounds of a lovely botanical garden, the Oregon Garden. If you stay in Portland these are all 'day trip' or maybe two-day trips on the western part of the state. I hope you have a wonderful time wherever you go!