Ba Sing Se

Ba Sing Se is the capital of the Earth Kingdom, which is split into the rich people section and the poor people section. The rich people section are ignorant of the poor people, and live very sheltered lives, carefully monitored by the officers. The poor people section is more fun, and the citizens are close and the community is tightly-knit. There are haiku parties, comprised mostly of girls, in which Sokka stumbled upon while doing something. They are very serious about haiku-ing. The scenery in the city is also really nice, except I kind of forgot it.
Omashu

Omashu is ruled by its own king, Bumi. Bumi is Aang's old friend from 100 years ago, and he is as insane now as he was before. He also has a pet goat gorilla named Flopsie. Although Flopsie has an intimidating exterior, he is actually as friendly as a friendly goat gorilla. Omashu has many chutes running through the town, used for transportation of products like food and clothes and everything. Omashu is the first time we see the Cabbage Merchant, when Aang is sliding down the chutes to run away from someone.

There is a legend that speaks of the origins of Omashu. There was once a man named Shu and a woman named Oma, who were from two towns at war, and they fell in love. Because they were not allowed to meet ever, they did their best to do whatever they could to see each other again. In their effort, they met badgermoles, and learned from them how to earthbend, becoming the world's first ever earth benders. With their new skills, they created a secret maze of tunnels through the mountains where love lit the way to their meeting place. However, Shu was killed in the war, and Oma was overcome with grief. She ended the war between the two cities, and they came together to form the city of "Omashu."
Kyoshi Island

Kyoshi Island was founded by the previous avatar Kyoshi, when she split away from the Earth Kingdom because of the corrupt government. The Kyoshi Warriors are the first people that Aang, Sokka, and Katara meet on their journey to the North Pole. They are a group of women who defend their island without the ability to earthbend.