Adding Water
Water is fun. You can have rivers to cross, lakes with objects on the top, or under the water . You can create water that is pitch black and looks like tar, or bright orange and looks like lava or even make a custard stream.
In fact you can have an entire water world with all the action taking place under water, and the objects you program are things like boats and submarines and fish.
The way water works in Kodu is it will cover all of an area that is a certain height. So if you add it to a new flat world the entire world is covered in water. If you build hills around the edges of the flat area then add water then it will stay in that one area between the hills - in the valley - of course if there is a small gap in the hills then all the water floods out and covers the rest of the world.