Here's a couple of links that might help you decide:
The city of Portland suggests three bike routes to the coast. I recommend the dotted green route.
http://www.portlandonline.com/transp...x.cfm?c=39889&
Alternatively, consider Hwy 30. It's busy and Industrial the first several miles. Then it becomes very rural and scenic but still a lot of traffic on weekends. Here's a map:
http://maps.google.com/maps?f=d&sour...471924&t=p&z=9