Here is the complete list of the possible rewards from LOL Mall Arcade.
Pet Prizes
PokerFace Happycat
Tagnol,evil brother of Longcat
Ya Rly Owl
Mystery Chests
If you hold these chests in your inventory long enough they will open (the image will change) and out will pop and exiting suprise!
Green Mystery Chest
Red Mystery Chest
Blue Mystery Chest
Brown Mystery Chest
Silver Treasure Chest