Pokémon Ruby and Sapphire/Slateport City

Slateport Beach
As soon as you arrive you will be in Slateport's beach and will encounter more than a couple trainers on your way up to the city. Talk to the little girl with the pink inner tube and she will give you a Soft Sand just for talking to her. The trainers aren't a big deal but will offer some experience points which will be useful in your next encounter with Aqua/Magma.

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Seashore House
At the north west part of the beach is Seashore House. The owner of this house loves watching heated battles, and if you can defeat all the trainers in the house, you get a special prize!

Deliver the goods!
With a busy market, a club, a museum and a contest hall(a battle tent for Emerald players), there's a lot to see and do. You still have to deliver the Devon goods, so go to the shipyard to look for Stern.

Where's Captain Stern?
The shipyard is where new ships are built and old ships are sank. Stern isn't around and the man reading the blueprints will confirm this. With nothing else to do, head outside and north to the Oceanic Museum.

The Oceanic Museum
When you first arrived, you might've noticed the long line of Magma/Aqua grunts waiting outside, blocking the entrance. Once you've spoken to the man at the shipyard, the line will clear and you can finally enter. The admission fee is 50, though if you have less than that the receptionist will think you are a grunt and let you enter for free. You'll immediately notice Team Magma/Aqua grunts all over the museum, and the first one you meet is the same grunt you battled before! He'll give you TM46 (Thief) so that you'll leave him alone. Thief is an interesting Dark Type move that let's you steal the opposing Pokemon's held items, but it's a pretty weak move.

Walk upstairs and you will meet Capt. Stern. After finally giving him the Devon goods you'll be confronted by two grunts who'll challenge you to a battle.

In Ruby


 * Team Magma Grunt


 * Team Magma Grunt

In Sapphire/Emerald


 * Team Aqua Grunt


 * Team Aqua Grunt

Once you defeat the grunts their leader will show up. After warning you not to meddle in their business any further, they will leave.

Name Rater
The Name Rater lives in a house in below the Contest Hall/Battle Tent. He will offer to rename your Pokemon, unless it's obtained from trading.

Slateport Market
The Slateport Market contains a myriad of items that are not found in a typical Poké Mart, so it's well worth your time to visit.

Doll Vendor
The doll vendor sells dolls that you can use to decorate your room or secret base.

TM Vendor
This vendor only appears after you've received TM43 (Secret Power) from one of their members on Route 111.

The Energy Guru
The Energy Guru's items might be pricey, but they're very useful. Each item permanently increases your Pokemon's stats. Also, the woman to his left will give you an effort ribbon if your main Pokemon doesn't already have a ribbon and had accumulated 510 effort values.

Secret Base Shop
This vendor will only sell you items after you've received TM43 (Secret Power) from one of their members on Route 111.

Hyper Contests
At the northwest part of Slateport in Ruby and Sapphire is the Hyper Rank Contest Hall. Your Pokemon aren't egligible to enter this contest just yet, but you can get some items here. Talk to the girl and she will give you a Pokeblock Case, which let's you store Pokeblocks that you can make with berries using machines in the Contest Hall. A sailor near the stage will give you TM41 (Torment) for listening to him. It prevents a Pokemon from using the same move twice, and can be pretty devastating for Pokemon with only one or two damaging moves.

As for entering the Pokemon Contest here, you can only do that if the pokemon you're entering has beaten the Super Rank Pokemon Contest in Fallarbor, which is still quite far away.

The Battle Tent
For Emerald players, the Contest Hall is replaced by the Battle Tent. There is also a sailor there that will give you TM41, so be sure to talk to him.

Battle Tents always contain some kind of gimmick that is used in the Battle Frontier accessible only after you defeat the Pokemon League. Battling here gives you no experience or money, though you do receive a prize if you complete the challenge.

The gimmick of this Battle Tent is that you can't use your own Pokemon. You have to select 3 Pokemon from a random selection of 6, all at level 30. You can check their summaries to know their stats and moves, and it's advised that you do that. Once you've selected your Pokemon, you'll battle against 3 trainers using random Pokemon. After each battle you can choose to trade a Pokemon with the other trainer, though you can't see their summaries and have to deduce their movesets from observing them in battle. There's not much of a strategy to help you beat this Battle Tent. Just choose a Pokemon with a good moveset for your team and hope you don't get put at a type disadvantage against the opponent's Pokemon. You get a Full Heal for beating the challenge, which let's you heal any status afflictions on one Pokemon. It's a very good item, so it might be worth it to take the challenge multiple times.

The Pokemon Fan Club
The Pokemon Fan Club is a club where trainers meet and chat about their favourite Pokemon. Talk to one of the woman and she will give you a Soothe Bell if your Pokemon has high happiness. A man will give you a scarf if any of your Pokemon's contest stats are maxed out. He'll give a different scarf for every different stat.

Items

 * Great Ball: Hidden South west of the Seashore House, under the blue towel laying on the beach.
 * Ether: Hidden on the south-east corner, directly south-east of the two children, and east of the beach towel.
 * Revive: Hidden at the southern portion of Route 109 beach. It is one square north of Mr. Briney's landing point and five squares west.
 * Heart Scale: Hidden south of the Seashore House, under the green parasol.
 * Heart Scale: Hidden one square north of the old man with his Zigzagzoon.