There are quite a few parks you can hit up in Seattle (or well, the surrounding area). It also depends on what kind of area you want..Depending on the character, some places may look odd for a picture..Like, let's use me for example. I could go to a park in my AC Cloud cosplay, but Cloud wouldn't be in the see-saw area..He may be more by a pond or a lot of trees..
What character or series (if going in a group) are you going as? And what kind of things can you see in the background (picturing in your head of course) of a picture they are in? If you can answer these 2, I can give you specific parks or locations (I'm born and rasied in king county so that might tell ya somethin lol)