All of the autumn events could be reused or recreated with a spooky twist to make them more Halloween-y (like a Halloween maze, candy hunt, pumpkin harvest, trick-or-treat race, hide from the monster, etc.). There could be cash or object prizes for 1st, 2nd, and 3rd place, or they could just be for fun.
For those players who can't participate when the events happen (probably 3pm/9pm EST), there could be smaller events that happen every hour (?) administered by staff, police, arch, or any dedicated player. These events would be much simpler and just for fun, not money. Some events would be candy drop parties, house decorating, skin comps, musical chairs with spooky music, haunted house tours, etc.
The build comp should begin in mid-October with the events lasting from Oct 24th-30th. The winning build could be displayed and a server-wide trick-or-treating event could take place on the 31st