Seattle (and the surrounding suburbs) are a big place. The quality of public transit really depends on where you are staying and where you want to visit. The inner-most downtown area is relatively small and walkable assuming you are okay with some rain. The bus system is decent. If you want to go further out though it could be 30 minutes to an hour between buses.
What sort of subjects do you want to photograph? Any locations in mind?
I'll be travelling light with 35mm so I'm thinking of walking around town if there are many nice architectural or human subjects otherwise I may just take ferry to Victoria BC and burn more frames there. I saw some nice pictures of the town.
I'm also interested to see Olympic park but only have 3 days I don't think it will be enough.