The Klickitat / Goldendale route is great!
Another, shorter one is to take I84 east to Corbett then Hwy 30 through the gorge to Hood River. You'll pass lots of waterfalls, good opportunities for short hikes and resting. Then take Hwy 35 south around mt Hood and go west on 26 to Government Camp. Go up to Timberline Lodge for outstanding views and explore the historic lodge (good place for lunch too). Just make sure you go to the TIMBERLINE LODGE not the WYEASY DAY LODGE. Then take Hwy 26 back to Portland.
You mentioned Astoria ... to be honest, that isn't the best loop. Hwy 30 to get there is sort of boring, and so is Hwy101 from Seaside to Astoria (plus it is full of traffic). If you're determined to go that way, consider 26 west to 47 through Vernonia and Clatskanie, then take 30 the rest of the way to Astoria. Or consider 202 from Vernonia to Astoria. It's a very tight and windy road but watch for logging trucks ... in the past the pavement hasn't been the best, dunno what it's like now.
I was going to suggest some coast loops but I think you'll get your share of them on your way home. HWY 26 will take you to the tourist trap of Cannon Beach, but Hwy 6 is much more fun and drops you off in Tillamook (and the Tillamook cheese factory). From there go to Cape Meares (lighthouse) and continue past Lookout Cape and Cape Kiwanda. Another way to get to the coast is Hwy 34 from Corvallis to Waldport.