I went to Next Fall 1 hour before curtain on a Wednesday mattinee and there was nothing left, and the guy was really rude (not part of your question, but it really ticked me off). I would try to get there at least 2 hours before for that one.
Still nothing for Come Fly Away, but it'll be on TKTS if that's in your price range.
"I've never encountered such religiously, you know, loyal fans as Broadway musical theater fans. It's amazing."
--Allison Janney