With a week you can spend a couple of days in each city and a couple more visiting some nearby natural attractions. I'd recommend planning an "open jaw" route, flying to either Portland or Seattle, then returning (presumably to NYC) from the other. The train between the two is affordable and scenic.
You'd have a wide, wide choice of things to do and see both within and outside the two cities - farmers markets, ferries, the Space Needle (your "observatory") in Seattle... the list is too long to itemize here; look at the "top questions" (to the right) on both the Seattle and Portland boards.
For outings, I'd recommend Puget Sound and Whidbey Island from Seattle (other excursions need more time) and the Columbia Gorge and Mount Hood from Portland. Both are visited either as day trips or with one overnight; a car will be needed.
Both towns are foodie meccas, but much depends on your personal tastes. Time to do your homework.
Edited: 2:35 pm, February 09, 2014