In Canalave City, you can challenge the Steel-type trainers of Canalave Gym.
Go northwest and you will battle Black Belt Ricky, who has a level 33 Steelix. You get 1056 Pokédollars for winning.
To the east, you can battle Worker Gary, who has a level 31 Onix. You get 1488 Pokédollars for winning.
There are four moving platforms. If you use the leftmost platform, you can battle Worker Jackson, who has three level 28 Onix. You get 1344 Pokédollars for winning.
Back on the first floor, if you use the rightmost moving platform, you can battle Ace Trainer Cesar, who has a level 30 Skorupi and a level 32 Steelix. You get 2176 Pokédollars for winning.
Go west from the Ace Trainer and use the moving platform there, then go to the left and step on the moving platform, then go west and use the moving platform to go up one floor.
Go north and you will battle Worker Gerardo, who has two level 29 Onix. You get 1392 Pokédollars for winning.
Go east and you will battle Black Belt David, who has a level 30 Onix and a level 32 Steelix. You get 1024 Pokédollars for winning.
Go south and use the moving platforms. You will go down one floor, where you will battle Ace Trainer Breanna, who has a level 33 Azumarill. You get 2244 Pokédollars for winning.
Go southwest from the Ace Trainer and use the moving platform there. Then go west and step on the moving platform. Then go up and talk to the Gym Leader to start the battle.
Gym Leader Byron
Byron has a level 36 Bronzor, a level 36 Steelix, and a level 39 Bastiodon. You get 4836 Pokédollars for winning. You get the Mine Badge, which lets you use Strength outside of battle. All pokémon up to level 70 will obey you. You get the stickers that Byron uses. You also get some TM91s, which teach Flash Cannon.
Go east and use the moving platforms to return to the first floor, and go south to leave the gym.
Go to the Library
After you exit the gym, follow your friend to the Canalave Library. In the library, go up to the top floor.
After the cutscene, go outside.
Your next destination is Lake Valor. You can get there by either going south from Veilstone City or by going east from Pastoria City.