To set the scene, an Arctic Tundra or Winter Dream wallpaper will be the perfect backdrop to start decorating. The Arctic Tundra wallpaper has a limited quantity with just over 6,000 available as of this writing. There is also an Ice Slate Gravel for 29 Facebook Credits that has over 8,000 left.
For animals, the Ice Dolphin and Black Beluga are both pretty darn adorable and priced at 50 and 40 Facebook Credits respectively. Both of these are also limited, and the Black Beluga only has 1,000 left so you'll have to be quick if you want to have one floating around your aquarium. The Emperor Penguin is also a must have at 40 Facebook Credits.
Decoration-wise, you'll find tons of various ice mountains, sculptures, and glaciers in all different sizes and shapes. Many of these are available for coins, but there are a few that will require Facebook Credits. There is also an igloo for 230 coins which your fish will enjoy swimming in and out of.
This collection is large and fairly spread out, so it might take you awhile to find everything in the set. Make sure to check the "Specials" tab to find the majority of the Arctic items.