There is a little bit of cosplay that day. Its mainly just hanging out with people. There is normally a pool party at just about every hotel. Explore the city. You said it was your first time to sak, is it your first time in Seattle? If so Pikes Place is just a few blocks away and I recommend you visit it. Lots of cool things to see and great food. If you have a car check out the space needle and all that crap.
What I strongly suggest is walk around the convention center to get a good feel for the layout. It can be overwhelming for someone their first time there, again if you haven't been there before.
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