For a coast trip, I recommend taking Highway 26 from Portland, and then take the Highway 6 turnoff to Tillamook. In Tillamook visit the Tillamook Cheese Factory! Then proceed up Highway 101 and stop whenever your curiosity is piqued. There are lots of beaches and parks and vistas to check out. Some cute towns along that route are Cannon Beach, Manzanita, Seaside, and Astoria. If your kids are fans of the movie The Goonies, you’ll definitely want to check out the Oregon Film Museum in Astoria.
Returning to Portland, if you’re spent already, just take Highway 30 back. But if you still have time, I recommend taking highway 202 from Astoria past the Jewell Meadows wildlife viewing area – if you’re lucky, you’ll see herds of elk. You can proceed to Highway 26 which will take you back to Portland. You could also do this whole trip in the reverse direction, if you’d prefer.