Pokémon Ruby and Sapphire/Slateport City

Deliver the Devon goods
Go into the place with the boats out the front and talk to the guy designing ships. He'll ask you to deliver the goods to Capt. Stern, who you will find in the next part.

Now go into the building with the rocket people in front and get TM46 (thief) from the the grunt. Go upstairs to talk to Capt. Stern, then battle two grunts and you are done here.

Go into the contest building (the red building with the grey roof) and talk to the little girl inside and she will give you a Pokéblock case. A sailor near the stage will give you TM41 (torment) for listening to him.

Talk to the woman wearing a dress in the building next to the Pokémart and she will give you a soothe bell and a scarf if your first party Pokémon is in excellent condition (use the Pokéblocks).

For Emerald players: There is a new red building. There you can rent three pokèmon and fight different people.