Extra points
The penguins on the iceberg can be shot for extra points -- just click them. If you hit them, you will get 5 points. Hit them in the head for 10 points.
Sunk iceberg
During a single player game, you will see an iceberg at the right side of your screen. If you shoot enough, you should find a spot on the iceberg that you can put a hole in. Once this is done, the iceberg will sink.
Bird droppings
At the main menu, you will see a bird flying at the top of the screen. Click on it and it will crap. Repeatedly click on it and you will have have bird droppings sliding all across the bottom of your screen.
Shoot the penguins
In single player mode, you can shoot the penguins for 5 or 10 points each. However, in the final round the penguins slide down the iceberg and jump into the air. Shoot the penguins in the air for 50 points each.