First of all, there is Highway 6, on the way to the oregon coast from north plains to
Tillamook. Heck of a drive, on a lot of days it will be fairly empty because most traffic takes hwy 26. Beautiful curves, and an awesome view, one of my favorite roads to drive of all time.
Another fun road, but generally has a bit more traffic, but is much closer to portland, is Clackamas river rd., its off the Park place/mollala exit off of I-205 and drives along the clackamas, its a gorgeous drive and offers awesome corners, but is partially resedential in some areas so be careful. As you get further out it can take you into some fun country roads that are usually pretty clear for some spirited driving, and a usual lack of police .
Another fun drive is hwy 224 on the way out of town from portland. You take the Mt. Hood/Estacada Exit off of I-205, and veer right towards estacada at the split, some extremely fun and tight curves close to the clackamas, but houses are scattered along the road at this point so be careful, as you get further out, past estacada, you come into the base of mt. hood national forest, and the road is smooth with wide curves, and there are tremendous views of the surrounding hills and the clackamas river. As you get further along the road will split, and the smaller road off to the right is really fun with lots of little drops and divets taht toss you around, it feels like a rally course without the paint destroying effects of gravel, its way fun to drive on a sunny day.