First of all Avoid Interstate Freeways as much as possible. They are long, straight and boring and you miss a lot. Try to take the secondary roads and Scenic Routes. Only useful for getting from place to place quickly.

Part of the world I know best
Boise
Payette
Baker City
La Grande
Pendleton
Kennewick
Yakima- -Naches
Tacoma- -South Cle Elum
-North Bend
-Issaquah
-Newcastle
-Mercer Island

from Boise take 84 to Highway 20 into Vale Oregon, then take 26 to highway 19 into the John Day Fossil beds. follow 19 to Fossil then take 218 thru Antelope and into Shaniko. Near SHaniko look for the place you can see 4 snow capped volcanos from the same place.
From Shaniko take the local road toward Bakeoven and on to Maupin. Then take 216 to 26 and ride around Mount Hood, yake a short side trip to Timberline.
From Portland cross the COlumbia River and get on Highway 14 East to Carson then go North on the Wind River Road to the NFS Road 25, take 25 North to the turn off to Windy Ridge, take a side trip to Windy Ridge to see the Mount St Helens eruption area. Get
back on 25 and continue North to Highway 12 at Randle (good food at the Mt St Helens Inn there). Take highway 12 East to highway 123 (Cayuse Pass). North on 123 to 410 (Chinook Pass) g East on 410 up and over the top as a side trip then head back west and into
the Puget Sound area to Seattle. Avoid I90.

Or From Naches take chinook Pass, highway 410 toward Enumclaw. Simply stunning vistas and great twisties. Once you get to Tscoma forget I-90 Snoqualmie Pass to Cle Elum - boring freeway. No big deal and not worth
the gas.

I would suggest trading Madisonville for: Deals gap to foothills pkwy. then 321 through Maryville, Friendsville (where I live),Lenoir City, on to 70 west towards Nashville.
I'm told the cops on Cherohala are pretty tough on motorcyclists.
Scott

I found this on another posting if you fancy a taste of the wilderness. It's far from a northern route, but does look good value:

http://www.transamtrail.com/