Zoo Tycoon Easter Egg - Hidden Items and Halloween Colors

I can't take credit for theese as I found them in a website.

During October 31st on the in-game calendar pause the game. Now look in your availible buildings under scenery. You will be able to buy a Jack-o-Lantern.

During December 25th on the in-game calendar pause the game. Under scenery you can buy a snowman that won't ever melt! Under foliage you can now grow a Christmas tree.

During October 31st in real life(as long as your computer clock is set properly) all of your buildings and people will be orange and black!

Note: These will not last for the rest of the year.