Seattle in August

I will be attending a conference in Seattle next month, August 6-12. I plan to tack on some time on either end to play tourist. I'm arriving by air, and do not intend to rent a vehicle.

Are there any private gardens I may visit? Perhaps someone reading this will welcome a visitor. ;-) 

I will also look up public gardens, some of which I have visited in the past, but advice from locals is always the best option.