Well, it depends on where you're starting from, but there are quite a few pretty reasonable rides of varying distances and difficulties here at the Seattle Bicycle Touring Club maps library:

http://www.seattlebiketours.org/members/library.html

I've done a few of their routes and found them quite enjoyable. If you're just looking to toodle around a bit, then you might check out the King County Bicycle maps here:

http://www.metrokc.gov/kcdot/roads/bike/map.cfm

Good luck and have fun!