We use to have a server that had some fun plugins (I think server 6) that might still be and could probably be changed up if wanted.
The bong server where the majority of regulars went to go had various plug-ins. People had the preferences on each, but the plugins were:
Medigun Rubberband / Jumping
Engineer could place 2 dispensers, metal for dispensers and teleporters reduced and upgrade req. reduced.
Infinite ammo during waiting for players.
5 cap limit instead of 3
Player pushing/bumping removed on teammates and buildings, you'd now essentially noclip thru your teammates.
Jukebox, just a way of playing music in the server
They weren't really 'fun' plug-ins, they mainly tended to change up the playstyle of the game. If I had to pick one though, the flying medic would be it.