||They were added during the 10th Anniversary. Any monster with 70+ experience could drop them. They are not worth much, but can give Green Balloon, Red Balloon, 10 Cookies, Cream Cake, Party Trumpet, Bar of Chocolate, 3 Candies, Party Hat, Party Cake, Fireworks Rocket or Piggy Bank. They are once again obtainable by doing a quest.
From a sample of 447 surprise bags, there was no contradiction with the assumption that you can get each of the 11 items with 1/11 chance for each item.
See also: Red Surprise Bag.
Can be obtained in the Ultimate Challenges Quest on Greenhorn level, if you choice the present box as reward.