Multnomah Falls.... yes, beautiful... we camped there and sat at our picnic table in the dark as the woman in the site next to us undressed in the light of her car... dh got quite an eyefull that night. lol! Oh the memories.... that was a fun trip. Go up to Mount Hood - in the Lodge you can get a nice ploughman's lunch upstairs (at least we did in '96 on our 10th anniversary)with an incredible view of the summit of Mt Hood... and the stars at night up in the mountains are incredible!!
And definitely drive down the coastal side of the Cascades - gorgeous!!! Crater Lake is incredible.
(Ok, I just reread this and have to add that we didn't just sit there and watch that woman - it was just all of a sudden that she whipped her top off - we didn't have time to react before we saw it all! lol!!)
I've never been to Washington - yet. But we definitely want to go to the Olympia Rain Forest!