The Ballard Locks have some neat wave sculptures that might make good backgrounds, as well as a pretty little park with a rose garden and some grassy hills.
Gasworks Park on Lake Washington is fantastic for old abandoned machinery. You're not supposed to climb on it, but ...
The Space Needle makes a great backdrop, and is very iconic. The EMP museum right under it gives some weird and interesting shapes and colors
for backdrops, too. You can even go inside and shoot with their exhibits