After you get the Feather Badge from the Fortree Gym, you can continue along Route 120, where you got the Devon Scope.
In Route 120, in the long grass, you can encounter Oddish, Marill, Absol, and (rarely) Kecleon. In addition, in Ruby and Sapphire you can encounter Surskit, Zigzagoon, and Linoone. In Emerald, you can encounter Seedot, Poochyena, and Mightyena. If you Surf in this route, you can encounter Marill. In addition, by Surfing in Ruby and Sapphire, you can encounter Surskit. Surfing in Emerald, you can encounter Goldeen. If you fish with the Old Rod here, you can catch Magikarp and Goldeen. If you use the Good Rod, you can catch Magikarp, Goldeen, and Barboach. If you use the Super Rod here, you can catch Barboach.
Near the entrance of the route, you find Parasol Lady Clarissa, who in Ruby and Sapphire has a level 29 Goldeen and level 29 Wailmer, and in Emerald has level 28 Roselia and level 28 Wailmer.
East of the Parasol Lady, there are some stairs leading down to the water. Something is blocking the stairs, so check on it to make Kecleon appear. Catch it or KO it. Afterward, you can go down the stairs and get the Nest Ball on the right. Then you can Surf south to find a cave. Inside, you can get TM11 Sunny Day, which changes the weather in battle.
After that, leave the cave and go north again. You can go all the way north to find two trees that you can cut with Cut. Past those trees, you can get a Rare Candy.
Interviewer Gabby & Ty might be here. They have level 30 Magneton and level 30 Loudred.
Go southeast. When you go past the long grass, you find Bird Keeper Robert, who has Swablu.
After you defeat Robert, go west across the bridge, then go south. It will probably start raining here, if it hasn't already. To the south, you find Bird Keeper Colin, who in Ruby and Sapphire has a level 29 Natu and level 29 Swellow, and in Emerald has a level 28 Wingull and level 28 Natu.
Along the way, you find Parasol Lady Angelica, who has a level 30 Castform. North of her, in Ruby and Sapphire, there is Ninja Boy Tsunao, who has a level 28 Nincada, level 28 Koffing, and level 28 Ninjask.
To the east, you find Cooltrainer Jennifer. In Ruby and Sapphire, she has a level 31 Milotic. In Emerald, she has a level 30 Sableye.
South of Jennifer, there is a maze of tall grass. In the east side of the maze, you find Pokémon Ranger Jenna, who has Lotad, Lombre, and Nuzleaf (level 29 in Ruby and Sapphire, level 28 in Emerald).
On the west side of the maze, there is another invisible Kecleon that you can check on if you want to use the Devon Scope on it. Or you can just walk through the grass to avoid it.
Southeast of the Kecleon, there is Pokémon Ranger Carlos/Lorenzo, who also has Lotad, Lombre, and Nuzleaf (level 29 in Ruby and Sapphire, level 28 in Emerald).
Go southeast and jump down the ledge, then go farther south to find a bridge that goes west. Cross that bridge. After you cross the bridge, go south. To the south, you can see Bug Maniac Brandon/Jeffrey, who has three Surskit.
If you go down the stairs near the bug maniac, you can Surf north to find a couple of secret base spots.
Go south from the Bug Maniac. You will see some tall grass and a dirt path. You can follow that path to Route 121. Or you can go west and up some stairs to explore more of the area.