I would suggest a place like a park, so people wouldn't have to pay to get in. Could always do Washington park, since I think by now EVERYONE knows where that is ^^;
Another idea would be Holladay Park right outside of Lloyd mall, so could wonder into said mall for some more fun. Though, actually, I would probably rather Pioneer mall, since they seem more comfortable with people cosplaying. Lloyd has been having problems...
So there's another idea ^^
And what date are you thinking on having this? We could probably have it on the 28th, since most schools (at least Beaverton school) don't have school that day.