You know what? Here's a few ideas for this thread :
1) Jump Quest events
2) Riddle events
3) Scavenger hunt events (eg : Make a riddle clue to find the items you want, and first person to trade you with the items gets a prize)
4) Find-me events (eg : find the GM to get a prize)
5) Timed boss killing! (put a timer to kill a boss with your guild/party/w.e. and the team that kills the fastest wins!)
Those are some cool event ideas that you guys should do. ^^;
I'll update my post if I think of any others o.o
Oh, and also, I use to host "Jail Parties" in another private server where I was a GM. Like, Warp people to the jail and we summon monsters and bosses for the players. ^^