Traditional Twist
As a tribute to the original game, the chess pieces can be created to look like the ancient Indian military during the Gupta empire. Dress the king and queen in traditional Hindu dress, and have the bishops represented by elephants instead of horses. The horses can be changed to chariots, while the towers remain the same. While this is played the same as chess, the pieces harken back to the earliest game, known as Chaturanga.
Animals
Pit two different species of animals against each other on opposite sides of a chess board. Dogs and cats work well for this theme, as they can be shaped in different kinds for different positions, such as poodles for rooks, and a great dane for the king. The cats can be long hair cats for the knights and a siamese cat for the queen. Creating competing groups out of regional animals, such as African animals versus North American animals, can also be done. This is a great way to teach children about chess while maintaining their attention.
World War II
World War II chess pieces are perfect for the war history buff who also adores chess. Have the Axis on one side and the Allies on the other. Place Churchill as the king on the allies side and Hitler as the king on the other. The main second-in-command is the queen and the main buildings are the rooks.
Medieval Pieces
For those enchanted with the idea of dashing knights and battles of honor, play with a medieval themed chess set. For the rooks use castle towers, knights riding on horses, queens as the literal queen, and kings in their full army regalia. The pawns can be represented by archers or mere foot soldiers. Have one side in gold and the other in silver to differentiate them, yet lend a realistic coloring to the pieces.