Mrrzy, they've got you set up for 3 weeks at the least! If I were you I'd spend more time in the Seattle area and pick one direction for a 2-day side trip: North to Vancouver, east to the Grand Coulee if you insist (though I for one would never drive that far to see a damn dam), or south to Portland and the Columbia Gorge (where you can also see a damn dam). That way you'll have time to take a ferry across Elliott Bay, go sailing on Lake Washington or Lake Union (check out the Center for Wooden Boats at the south end), go to hear lots of wonderful music, (check out Victory Music for a massive list of all the folk music in the area), and RELAX -- you're on vacation! Besides, the twins will have much more fun going to the Zoo and Seattle Center (including the science museum) and the Ship Canal locks (kids love to watch the boats go up and down) than riding in the car for hours and hours. I have a six-year-old, so I know whereof I speak.

OK, you have all the advice, and everybody's favorite stuff. Now enjoy yourself! (One caution: Grand Coulee Dam and Bonneville Dam in "one day maybe"? No can do.)

Aloha,
Mark