I think your girls would really enjoy spending some time hiking a few of the waterfalls along the Historic Highway in the Columbia River Gorge. I spend a lot of time there with my girls (5 and 11). We also enjoy visiting Bonneville Dam and feeding the trout at the fish hatchery there.
Further out of the way, but really an adventure my family enjoys, is visiting the Painted Hills and John Day Fossil Beds in Eastern Oregon. It might be worth going out of your way on your return trip to visit. The kids can even dig for their own fossils behind the high school in the town of Fossil, then head over to the ghost town of Shaniko.