Pokémon Ruby and Sapphire/Fortree City

This is the location of the sixth badge. The city is full of treehouses.

Pokémon Mart
The Pokémon Mart contains the following items:
 * Great Ball: 600
 * Ultra Ball: 1200
 * Super Potion: 700
 * Hyper Potion: 1200
 * Antidote: 100
 * Paralyz Heal: 200
 * Awakening: 250
 * Revive: 1500
 * Super Repel: 500
 * Wood Mail: 50

Go up the ladder next to the Pokémon Center, you'll run into a girl inside the treehouse who wants a Pikachu.

In the next building to the right, an old woman will check your Pokémon for Hidden Power. She'll ask you to guess which hand she holds a coin in. If you choose correctly all three times, she'll give you TM 10 (Hidden Power).

Shop
Continuing down, you'll see a Shop on a tree. The right side sells chairs:


 * Small Chair: 2000
 * Pokémon Chair: 2000
 * Heavy Chair: 2000
 * Ragged Chair: 2000
 * Comfort Chair: 2000
 * Brick Chair: 2000
 * Camp Chair: 2000
 * Hard Chair: 2000

The left side sells desks:


 * Small Desk: 3000
 * Pokémon Desk: 3000
 * Heavy Desk: 6000
 * Ragged Desk: 6000
 * Comfort Desk: 6000
 * Brick Desk: 9000
 * Camp Desk: 9000
 * Hard Desk: 9000