I was just on decided to go tower diving one last time. That is one feature I felt was unique to this game, I will miss doing it, although it only grants an unseen achievement but one time. If you haven't jumped yet, now is the time to do it. I also checked and there is a total of 93 people in the Tower District. Obviously none of them are having any fun or even playing this area.
I also assume most don't know the area will be gone soon either, otherwise they would not be in there "wasting their time" on something that will soon be pointless.
I will also miss old Jerry in Blacklake. I should go visit that old spy. What will you be missing when module 21 arrives?