Hmm, I think that is not only a job for the adins/mods. Players could organize events too (or come up with ideas). They don`t have to be big events (they can be only for 2 hours or limited to a town if organized by that town, ...).
As an example for a typical weekend event:
Find a desert, mark a few plots (not towny plots) with torches, put up some chests full of sand and announce (2 weeks earlier maybe so people can fit it in their schedule) and host a competition about "who can build the best sandcastle in 2 hours".
(everyone is allowed to use 5 blocks of smooth sandstone in their castle to enable gates and opening that are not possible with sand alone).