While I was in Seattle for family and work, I had the chance to take a few photos I like. I went to two good locations: Seward Park and Gasworks.
At Seward Park, you can get a great view of Mount Rainier - but only when it's crystal clear, and only when you're using a good polarizer. If you get everything just right, your photo can look like this.
At Gasworks, the park is open until 11 PM, but it's completely unlit. You can't see where you're walking after dark without a flashlight of some sort. Through the miracle of long-exposure photography, however, you can see the gasworks lit only by the moon and stars. Gasworks also has, in my opinion, some of the best views of the Seattle skyline you can get. The reflections off Lake Union on a clear, still night are amazing. You also get a great sense of foreboding because you literally can't see anything but silhouettes walking around. Get the photos without losing your nerve!