If there's no panel I'm interested in at the moment and I don't feel a need for alone time, I tend to put on (or keep on) a costume and just hang out in a main area of the con, watching people (or not) in an in-character way, which tends to amuse people. Like at my last con, I spent some time dressed as Lunatic, hanging out by a balcony railing staring at the people coming up and down the escalators with my bowgun at the ready. I also spent some time as MakubeX, mostly ignoring people while I "programmed" (actually I was writing fanfic) on my laptop. Both made a few people do double-takes and come over to get a pic.
"If people are going to laugh anyway, I want to try to make people laugh, and then laugh myself."
-Emishi Haruki, Get Backers