Whenever we are down in the Gorge (Sportbike Northwest is held in Stevenson WA the end of August. See
www.soundrider.com) we go out to Sandy, turn off onto Ten Eyck Rd, to Bull Run to the Historic Highway but then if you have time go up Larch Mountain Rd to the overlook, Take some walking shoes because at the end of the road you have to hike about a mile up to the overlook. It is worth the walk!
Once you get back to the Historic Highway it will eventually dump you out onto I-84 but after a couple miles get off at Cascade Locks and go across the river (Toll Bridge but it is only .50 for bikes) and then east to Carson. That is where you turn north to get to St. Helens. If you Google a map from Carson to Randle it will show you the route but at about 60 miles take the turn off up to Windy Ridge. It is pretty amazing. The east side of the mountain was stripped bare of all vegetation when the volcano erupted but the way it is coming back to life is interesting to see.
If you have time to see the west side too the ride around the south end is the best route, and the trip is worth it.
ON THE OTHER HAND...
As far as central Oregon goes, pick up "Motorcycle Journeys Through The Pacific Northwest" by Bruce Hansen. He has covered it really well.
We did Hells Canyon last year and it was absolutely amazing. We came south from Pendelton and then east from Baker City through Hells Canyon. It was April and the Wallowa Mountain Loop Rd was still closed with snow so I am planning on going back over this September. If you do this ride go to Oxbow and then down river to the Hells Canyon Dam. It's about 20 miles on a little two lane road that is very nicely paved... with no guard rails. At one point I am cruising along about 20 feet above the level of the lake. I start blasting the twisties and the next time I look over it is a thousand foot drop to the water!
Anyway, the best way north from there is the Wallowa Loop Rd to Joseph, then Enterprise, take Highway 3 up to Clarkston WA. Highway 3 is kind of dull until you get about 5 miles from the WA border. That is when you start down into Rattle Snake Canyon. Part way down into the canyon you cross the state line. Notice the change in road engineering. It is very interesting.
From Clarkston / Lewiston you can take Highway 12 up over Lolo Pass (Make sure you stop outside Kooskia ID and get a picture of your bike next to the "Windy Road Next 75 Miles" sign) to Missoula, then up to Glacier. (Do the east side of Flathead Lake) It just goes on and on and on...