I've been up to Vancouver Island 4 times in the last 6 years. I went up the west coast of the island the first time and that is when I fell in love with the island. I promised myself that when I grew up I'd move to Vancouver Island
The only area of Washington State I haven't travelled is north of Spokane.
Other ideas: Isn't there an amusement park near Coeur d'Alene?
Up Hwy 97/97A to Chelan and beyond?
Banks Lake & Grand Coulee Dam?
Hwy 84 along the south side of the Columbia.