Romeo - You can ride the scenic part of the gorge in 1/2 of a day. Good roads north to south in Oregon are virtually nonexistent; there is 101 on the coast, I-5 down the valley, or 97 down the middle of the state, and 395 on the eastern side. East to west roads, there are a lot of good choices.
So my thought would be to ride up the south side of the gorge from Portland east on I-84 the Corbet exit following the old scenic highway east to Crown Point viewing area.

Continue east on the scenic hwy

where there will be a bunch of waterfalls.

Continue east on old scenic hwy, it will return you to I-84, continue east on I-84, exit at Mosier, continue east on the old hwy (It parallels I-84)

up to Rowena Crest to the viewing area there.

Continue east on old hwy where you can rejoin I-84 or you can stay on it all the way to The Dalles. Now you can cross over to the north side of the river or go less than another 20 miles east on I-84 to Biggs Junction and cross over the river there to the north side and ride back to Portland. If you do this, you will be able to go to Maryhill museum and Stonehenge and a really sweet winery. Then head west on Hwy 14 back to Portland. Then from Portland, take Hwy 30 west to Hwy 101 south all the way to Coos Bay taking Hwy 42 east to Roseburg, then Hwy 138 up to Crater Lake. OR instead of Hwy 42 at Coos Bay, stay on Hwy 101 to Crescent City then east on Hwy 199 to Grants Pass. Then take I-5 to Gold Hill, Hwy 234 to Hwy 262 and then up to Crater Lake. Hwy 199 is a longer route but it does follow the Smith River. Then from Crater Lake you have several options to head back east towards Michigan.
If you have questions just shoot me an email and I will give you my number so we can talk. Hope this helps.
Razkal & Lady H