Winter Garden Mystery Boxes with prices of 6 SimCash for one box, 30 SimCash for five boxes and 60 SimCash for ten boxes.
There are 24 Snowman Mystery Box Items to collect in total consisting of 4x Super Rare, 10x Uncommon and 10x Common. By collecting all of them you will unlock The Sterling Gramophone Music Skill Item. It doesn't however seem like The Sims Social developers have changed the way Mystery Boxes are given, as they are still completely random and the most off putting problem is that duplicates can still be attained.
Here is a list of all of the items available along with House Value, description and recommended SimCash price: