This whole thread might be a repost. I swear I've seen this asked before again and again. But I'm willing to help. I too have been in search and stumbled upon this site:
http://www.motorcycleroads.us/states/wa.html
Motorcyclists know their roads. I plan to drive all of them during my ownership of the BRZ. And drive them all again when the STI version comes out
If you want something quick go do the Mercer Island loop. Be carefull of pedestrians. That's the only worry I have but the banks and turns by East Mercer way is just awesome. It's like mountain driving in city levels. Enjoy!