The Columbia Gorge begins... or rather ends just east of Portland. This is a pretty dramatic section of the river and the roads on either side are both incredibly scenic. It's difficult to see away from the gorge on the side you're driving on but from the Oregon side you can occasionally see the peak of Mt. Adams over in Washington. From the Washington side of the river you get a few decent views of Mt. Hood in Oregon.
The Mt. Hood railway leaves from Hood River, Oregon and travels south to the town of Mt. Hood, Oregon which is not far from the peak of the same name. This is a nice scenic relaxing trip (you can ignore the fake train robbery going on elsewhere in the car). It is worth the extra money to sit in the dome car for the views alone.
The railway leaves you an hour or so to wander around the crossroads of Mt. Hood. This is where you will find Solera Brewing which may not be the most scenic brewery in the world but it's up there. If you'd rather, you can go get ice cream or souvenirs instead of beer but well, why would you do that?
Meanwhile, back at the river there are about a thousand people learning to wind surf on the flats facing the town of Hood River. This is a spectacular thing to see and yet we have no pictures of it. Instead we have a picture of Bonneville Lock and D...
Inside the dam visitor center you can take an elevator down to the bottom and see this view. These are lampreys and they are firmly attached to this window against the rather heavy current through the fish ladder.
You can also see more common fish like salmon at the fish ladder, or you can watch from above ground where you're sort of at the mercy of waiting for them to jump along steps of the ladder.
There are dozens of waterfalls along this route as well. Some of them are pretty strenuous hikes (especially on the Washington side). Others, like Multnomah don't even require getting off of the interstate. You can literally park between the two sides of I-84 and take pictures of Multnomah. Or you can cross under to the falls and optionally connect to a larger series of trails that go up behind the falls.