To give you a little behind the scene tour of seattle i will let you in on a few nice places, but you will want to make sure to keep an eye out for strange people in some of these places. Seeing as you are in the city with cosplay and nice photo gear. you never know when you could run into the wrong person. I love seattle and have lived there for a while, but i have also run across a few jerks too.
My fave place for photos is the Graffiti Free Wall in the Sodo dist. on S Stacy st and Occidental ave S behind the warehouse near the train tracks. it is a little hard to find but if you google maps that cross street and look to the East there is a warehouse that you can go behind to find it.
If you have ever been to sakuracon you know outside the back of the convention center you have freeway park.
Also if you follow the NE side of the park out the back you will find some steps leading down into an ally that eventually leads you under the park and to a little bit of a under bridge area that is interesting.
If you like steam punk look no further than Gas Works Park. This place has some interesting structures that would provide the perfict back drop. Not only does it have quite a few interesting places to shoot at, but it is also along the water. I enjoy going there during the winter when it snows (well the years it decides to snow in seattle that is...)
If your interested in the gum wall it is right under pike place! this is kinda a high population area so it would be a little odd trying to get a photo shoot there.
Want a nice beach with a city view. Check out Alki Beach in west seattle!
Back to a better part of town. Volunteer park water tower! This place is amazing. its an old rustic looking castle thing. I know it is a water tower, but it looks so much like a turret that should be on the side of a castle. The inside has a long staircase that winds around and up to this crazy looking room with benches, chain link fence, steel cover for the water tower and slits in the walls as the windows. you have to see it if you havent been there before.
Another odd place is Marsh island! get a look at the wetlands under a freeway that are turned into a park!
West Point light house. This place is a must for any one! hell i would like to see any adventurer in front of this place. Like Edward and alphonse elric would be great in a place like this!
Now if you are an adventurous photographer and dont mind a little tresspassing there are a few other places that i know of, but i will not list them on here. if you want more pm me and we can talk on email or something!