After passing through Vallée Étroite Way and its entrances to Terminus Cave, you go south to Couriway Town.

Explore

Turn on your Dowsing Machine and go south. You'll find a hidden Pretty Wing on a bush southwest of the Pokémon Center.

Go south from the Pokémon Center, then into the house to the west. If you have a pokémon in your party that knows Nuzzle, the girl will give you a Revive.

Turn on your Dowsing Machine and go up the stairs east of the pokémon center to find a hidden Ether in the corner.

Go up the stairs to the east and use your Dowsing Machine to find a hidden Poké Ball between the benches.

Go to the right of the benches to get a Rare Candy.

Talk to the old woman on the train platform to get TM89 U-turn.

Go down the stairs, then go east onto the wooden platform. Use your Dowsing Machine to find a hidden Burn Heal in front of the lower bench.

Battle the Professor

Go west from the wooden platform, then go south toward the bridge. Professor Sycamore appears. He challenges you to a battle. He has a level 50 Venusaur, a level 50 Blastoise, and a level 50 Charizard.

Continue Exploring

Go south along the bridge, then go into the house to the left. The girl in the middle of the house will quiz you about your starter pokémon. She'll mention a height and a weight and will ask if those are the correct height and weight of your starter. Here are the correct answers:

If you get the correct answers, you receive a Mystic Water if you chose Froakie, a Miracle Seed if you chose Chespin, or a Charcoal if you chose Fennekin.

Go east from this house, then up the stairs. In the Hotel Couriway, talk to the girl on the right side. She'll ask you a quiz question based on information from the Pokédex. If you give her the correct answer, she'll give you TM55 Scald.

Go upstairs. In the room on the right, you'll find Mr. Bonding, who gives you the fifteenth O-Power: Stealth. It reduces how many wild pokémon you encounter. It's like using a Repel.

Back in Couriway Town, talk to the guy east of the Couriway Hotel to buy some Fresh Water. He also gives you a free berry.

You can walk in the shallow water behind the Fresh Water seller. Use your Dowsing Machine to find a hidden Prism Scale on the small square island here. If you trade a Feebas that is holding a Prism Scale, it evolves into Milotic.

Get the Max Potion that is in the corner to the right of the three small waterfalls behind the Fresh Water seller.

You have explored everything in Couriway Town at this point, so go south from the bridge to reach the gatehouse to Route 19: Grande Vallée Way.