The Dalles is the east side entrance to the Columbia River Gorge.
Cross over to the Oregon side of the river and drive West from the Dalles. There are plenty of places to stop and check out waterfalls. Getting there is interesting as you head out 90 East through the mountains. The views are quite nice through there. But after that it's pretty dull until you get to the Gorge. There is a cool wind farm along the way, but the Yakima Indian Reservation is a really boring stretch of land.
If you're going to drive down through Portland anyway, you should definitely plan to see the Columbia River Gorge.