The Avatar and Bending Arts
As told by those who know it best

Calligraphy
The ancient writings in Avatar

Items - Weapons
Keeping track of all their stuff

Creatures
The animal hybrids that inhabit their world

Games/Sports
Even Avatar has its athletic/strategic side

The Spirit World
Where the eternal spirits reside

Geography
A look at their world

Avatar Temples
Sacred memorials for the previous Avatars