A commenter explains:

Elevator surfing was an activity that filled some college evenings when cash was scarce. Since you asked about how...

The largest challenge is getting into the elevator shaft. This can be accomplished through the obvious methods of breaking into an access room or by using an elevator key to open the doors. We were fortunate enough to find such a key. Once inside you need to get on top of the elevator. The easiest way to do this is to climb up supporting steel structure to the 2nd floor and wait for the elevator to come down to floor 1. Then just hop on quietly so you don't get caught. Other methods are more harrowing. things to watch out for are the counter weight and the obvious things like sticking body parts out while the elevator rises or falls. Since most elevators have controls on top, you can raise or lower them, stop them or otherwise overrule the floor selections of the passengers. Stalling elevators and then sliding down the supporting cables is fun, but can generate enough heat to burn your hands or other parts that you might foolishly wrap around the cable. Depending on the occupants state of intoxication, overruling their desired floor selections and/or stalling the elevator can be quite entertaining.

This must be the most counterproductive sign since "ABSOLUTELY NO LIQUID NITROGEN SALES TO MINORS".

Previously, previously, previously.