This is all Pre-9/11:

I knew some Qantas pilots that had a small blow-up globe that they pretended to use for navigation when someone visited the flight deck. They used to call it "the parade" because lots wanted to visit up front on long flights.

Some UAL pilots had one of those fake plastic urinals with the suction cups that they fastened to the 727 bulkhead in order to mess with new F/A's visiting the cockpit.

FedEx used to allow all employees to ride on the jumpseat as a benefit (maybe they still do). The protocol was that the jumpseaters would bring cookies or cake as an offering to the crew.

It used to be that depending on conditions and the crew etc. the captain would arrange for wife/son/girlfriend to sit in the jumpseat for the trip. If an FAA inspector didn't board and the door was closed, and you knew your crew, then maybe it was time for the F/A to do her first takeoff.

When most airplanes had two pilots and a flight engineer (waiting to upgrade to first officer) seat swapping to give the FE a leg was not terribly uncommon.

UAL lost a DC8 freighter circa 1982-3 due to seat swapping at Detroit - the elevator trim was left in the landing position when the takeoff was initiated. Ferris later said that if it had been a pax flight the airline could have been....

Post 9-11:

There have still been instances where the Captain pushes the system to allow a non-crewmember to ride upfront. This is a good method for a career change.

Kelleher always liked to ride up front so he could smoke.