Nightmare Fuel, no contest, it's a must-see. A couple causal fans I know have ranked it the best thing at the event. Unfortunately that's the one that requires a time commitment. I haven't heard of Marathon actually closing due to crowds (if it didn't last night, it won't). You can walk right in at showtime.
My guess is crowds will take some time to build Tuesday, so maybe 9 - 12 will be worst? But Tuesday opens so rare, and this October proven so busy, it's hard to tell. If you don't have Express, I'd prioritize what you want to do, to ensure you at least hit the highlights. If it ends up being slow-ish, you'll have time to make it further down the list.