Well, that's pretty easy: Go down the Pacific Coast! You can follow the ACA Pacific Coast Route!http://www.adventurecycling.org/routes-and-maps/adventure-cycling-route-network/pacific-coast/
With your timeframe, you can easily accomplish the route with 50-60 mile days.
As for getting to the Oregon Coast from Portland, here's a pertinent link to an old forum post that contains other pertinent links to other older forum posts:http://www.adventurecycling.org/forums/index.php?topic=9240.msg46341#msg46341
I do recommend taking the 202/47/Banks-Vernonia trail from Portland to the coast. It is very rural and very quiet in terms of traffic and people. It drops you off in Astoria, the northernmost point on US 101 on the Oregon Coast.
Links to maps:http://www.portlandonline.com/transportation/index.cfm?&a=316549&c=36638http://www.portlandonline.com/transportation/index.cfm?&a=316550&c=36638
I've never taken 30 the full way from Astoria to Portland, just the portion from Portland-Rainier and Clatskanie-Wesport, so I've missed the worst parts of US 30. But even that little bit makes me prefer the 202/47/Banks-Vernonia routing.
There aren't many services, but there's just enough. Make sure you're stocked when leaving Astoria, as there isn't another full service grocery until Vernonia, about 70 miles down the road. Only a couple of country markets with short hours in between.
The route from Astoria to Hillsboro (Portland westside suburb) is 100 miles. You can break it into two days by camping at Big Eddy County Park which is about 60 miles in. Few, if any, lodging options are found on this route so if you are doing a credit card tour you should be prepared to do the 100 miles in a straight shot. Possible, but there are two small passes to contend with on the route.
There are also two bus options from Portland to the coast: The NW Point bus to Seaside/Astoria, or the Wave to Tillamook.