How about every now and then, a random set of "collectables" (could be things like special posters on walls not usually there, or perhaps the old collectable from Ballrace hidden around in random places) appears in the lobby for a short time, and then disappears after a set time? Each player in the lobby who wanted to participate in the activity of finding these collectables could be rewarded Units based off of every one they find, so long as every one of them is found.
To recap, there could be a random event to hide collectables around the lobby, where each player is awarded for how many they find at the end if all collectables are found. If the players take too long, the collectibles will disappear and nobody will earn an award. Special messages could be printed in the chat or on screen to signify the start of the event, the time left, how many collectibles have been found out of the total number of collectibles, and the end of the event.
This would be a fun little scavenger hunt to get the entire server involved, as well as encouraging exploration around the map.
EDIT: 1 month later...