There are some incredibly gorgeous places along the Columbia River Gorge, as I recall.

St. Helens is not that far. On a clear day you can see it from Portland, or you could drive up and visit it. In certain spots you can actually see three volcanoes from Portland: Hood, St. Helens, and Adams.

There's a decent view of the area from the top of Mt Tabor Park, a tiny little eroded volcano in the middle of the Eastern suburbs of Portland.

Decent Art museum downtown.

My favorite part of driving to Portland is going Northbound on I-5 at night. When you reach Portland proper (assuming you stay on I-5 north and don't head directly into downtown), there's a stunning view of downtown and the river as you crest the top of the Marquam Bridge. It's quite breathtaking, especially at night, because the I-5 journey up until that point is mostly suburbs, and you don't quite get to see downtown or the river much at all until you hit the bridge. Then once you're on the bridge, it's like *BAM*, hello Portland.