While in Portland, you can see Multnomah Falls. If you drive the access road to the right of eastbound I-84, you can stop at other smaller falls and sights. You can also go to Mount St Helens for the day.
If you decide to travel east in Oregon, the John Day area is known for its rock formations and fossils and the painted hills nearby. Lots of hiking. And if you were there, I highly recommend a stop at Kam Wah Chung. It's a historic site related to Chinese immigration. Kam Wah Chung is a brief stop, only an hour or two, so not likely something you'd make a special trip to see.
If you're interested in the Oregon Trail, Baker City and Oregon City are good places to learn the history. Oregon City was more museum-oriented. Baker City was more outdoors, walking paths down to the ruts left by wagons.
Over on the coast, Boiler Bay and Depoe Bay were good whale locations. If you see something about Cape Arago State Park, know that trails and areas can be closed off April-June to give the sea mammals space and quiet for birthing and nursing their young.
Oregon Dunes
Sunset at Bandon:
When I drove into Bandon, the air was heavy. It looked dark, smoky, ominous, like a time I wouldn't want to be on a coast. I understand that condition portends these amazing sunsets.
Mussel garden:
Sea world when the tide is out:
Sea anemone having crab leg for breakfast: