After you use the Secret Medicine on the Psyducks blocking the way, you can reach the northern part of Route 210.
Pokémon in the Route
In this part of the route, when walking in the grass, you can find Bibarel, Hoothoot (night time only), Machoke, Machop, Meditite, Noctowl (night time only), and Psyduck.
When fishing with the Good Rod, you can catch Barboach and Magikarp. When fishing with the Old Rod, you can catch Magikarp.
Explore the Route
Go west and then north to find a Super Repel.
North of where the Psyducks were blocking the route, there is a hidden Hyper Potion.
West of where the hidden Hyper Potion was, there is a hidden Ether.
North of the hidden Ether, there is a hidden Ninja who will jump out if you come near. Ninja Boy Brennan has a level 25 Zubat and a level 25 Skorupi. You get 500 Pokédollars for winning.
East of Ninja Boy Brennan, Ninja Boy Fabian is hiding in the grass. He has two level 23 Geodudes and a level 26 Geodude. You get 416 Pokédollars for winning.
North of the Ninja Boy, there is a Hyper Potion.
Northwest of that, there is a hidden Full Heal.
To the east, a Ninja is hiding in the grass. Ninja Boy Bruce has a level 27 Stunky. You get 432 Pokédollars for winning.
North of the Ninja, there is a hidden Burn Heal.
North of that, in the northeast of the tall grass, there is an Ultra Ball.
To the north, the area will become foggy. You can get TMs for Defog by talking to the Ace Trainer near the start of the Great Marsh. The entrance to the Great Marsh is in the northern part of Pastoria City. You don't need to teach Defog to a pokémon to use it in the wild. Just receiving the TMs is enough to add it to your Pokétch, which you can use to remove the fog from the area.
To the north, there is a hidden Ninja. Ninja Boy Joel has two level 23 Zubats, a level 25 Skorupi, and a level 25 Golbat. You get 500 Pokédollars for winning.
To the west, you will battle Ace Trainer Alyssa, who has a level 27 Ponyta and a level 27 Grotle. You get 1836 Pokédollars for winning.
To the northwest, you can battle Double Team Jen and Zac. They have a level 27 Gyarados and a level 27 Raichu. You get 13392 Pokédollars for winning.
Go north up the stairs and you can battle Ace Trainer Ernest, who has a level 25 Mothim, a level 27 Luxio, and a level 26 Onix. You get 1836 Pokédollars for winning.
Go west. You can use your bicycle to ride along the narrow beam of wood to the west. At the end, there is a Ninja hiding in the dirt. Ninja Boy Davey has a level 27 Dustox. You get 432 Pokédollars.
South of the Ninja Boy, you can check on the rocks to destroy them with Rock Smash, then go west to get a TM30 Shadow Ball.
Go back the way you came and go south along the wooden bridge. You will battle Black Belt Adam, who has a level 29 Machoke. You get 928 Pokédollars for winning.
Go west, then north down the stairs, then east. There is a Ninja hiding in the grass. Ninja Boy Nathan has a level 25 Croagunk and a level 25 Golbat. You get 500 Pokédollars for winning.
To the east of the Ninja Boy, there is a Smoke Ball.
Go west and you will battle Bird Keeper Brianna, who has a level 27 Hoothoot and a level 27 Noctowl. You get 1188 Pokédollars for winning.
Go west and you can battle Veteran Brian, who has a level 26 Buizel, a level 26 Machoke, and a level 26 Girafarig. You get 2288 Pokédollars for winning.
To the north, there are berry trees.
Go south, then west, to reach Celestic Town.