In response to Flux's suggestions, which are excellent, you may want to try the other side of the Gorge, from Washington Hwy 14.
Start this tour by exiting I-84 at Biggs and taking US 97 across the river, climb the steep hill towards Goldendale, but only about 4 miles up or so you come to Hwy 14. Take a right and see a replica of Stonehenge set high on the side of some hills, the views are excellent. A short drive west is the Maryhill Museum. Another great view and picnic spot is there. The museum is famous for its Rodin Sculptures, for some reason, they have a bunch of them and a nice explanation of the lost-wax process. Also a wild modern "landscape" installation, but it is a bit sagging due to poor workmanship, but cool nonetheless.
Hwy 14 is two lanes, but scenic. As you head west, you will drop down to the Columbia river side. The highway (and the railroad next to it) have an amazing number of tunnels.
If you want to detour, you can take the road to Trout Lake. Right outside the town is an excellent view of Mt. Adams, one of the mighty Cascade volcanoes. After that, you may want to pick up a Gifford Pincot Forest Map, because there are lots of sights rolling on the backroads up there. Don't worry, most are well marked and fine for 2WD (I drive a Civic). Up there are ice caves, wide open lava fields, lakes. But the ice caves and lava tubes are especially special.
Here are some links to the ice caves:
http://www.a2zgorge.info/area/GulerIceCaves.htm
and the lava tubes a.k.a the "Ape Caves":
http://www.fs.fed.us/gpnf/recreation/ape-cave/index.shtml
You will be pretty close to the south side of Mt. St. Helens at this point. And there is devastation on this side to bear witness to, although the big blowout is facing north.
At this point, it may not seem like it, but you are pretty close to Portland.
You can connect back to "civilization" by taking WA 503 or come out at Battleground, WA. This area is rugged and beautiful.
If you have less time and just want to stay along the Columbia River, just stay on the 14 and see the stunning Bridge of the Gods and the Columbia Gorge Museum, both outside Stevenson. Further along is an excellent short but steep hike up Beacon Rock. The views are amazing, including Bonneville Dam. After that continue along 14 towards Camas and on into Portland, stopping at some more of the viewpoints.