Make a church and every Sunday get all the players on to go there and hold a 10 minute service where you sing and stuff.
Make a area/house with tons of secret areas and places, then hold a treasure hunt there and hide the chest in the best place you can think of.
Oooh, that's something cool that I've never seen done in minecraft before. And now of course as I google it, plenty of people have done it in minecraft before