Cross the Columbia to Carson Washington, hit Wind River Road, then Forest Road 25 to Randle. Forest Road 99 is a good sidetrip to Mt. St. Helens. Escape back to I5 using Highway 12.
North stretch of FR25 is bumpy.
No problem if you have an Esprit....
Conceivably, you could take Hwy 12 to Yakima, then south to the Columbia again, hit Rowena loops, then back to Portland.
To avoid coppers, do FR25 on a weekday. in the summer or autumn.
Pic shows Rowena near the top.