After going through Route 120 and encountering more invisible Kecleon on the way, you continue southeast to Route 121.
In Route 121, you can catch Oddish, Gloom (rare), Wingull, and Kecleon (very rare). In Ruby and Sapphire, you can catch Zigzagoon and Linoone. In Emerald you can catch Poochyena and Mightyena. In Ruby, you can catch Duskull, and in Sapphire and Emerald you can catch Shuppet.
By Surfing, you can catch Tentacool, Wingull, and Pelipper. If you fish with the Old Rod, you can catch Tentacool and Magikarp. If you fish with the Good Rod, you can catch Tentacool, Magikarp, and Wailmer. If you use the Super Rod, you can catch Wailmer.
Near the entrance of the route, you find Hex Maniac Tammy, who has a Ralts and Duskull/Shuppet. In Emerald, she might join Bug Maniac Cale in a double battle.
North of the Hex Maniac, there are berry trees.
If you go south of the fence and walk east, you can find a hidden HP Up.
Go east between the fences. You will pass by Beauty Jessica, who has Kecleon and Seviper.
If you cut the tree to the east, you can reach the Safari Zone, where you can catch different types of pokémon.
Otherwise, go south down the stairs. You can cut the tree to the west to reach a secret base location. Farther south, there is a dock where you can Surf to access Route 122. You will go to Route 122 soon, but you should go to the next city first so you can Fly back to that city later.
Go east through some grass. If you go up the stairs, you will pass Sr. and Jr. Kate & Joy. They have Spinda and Slaking. If you keep going north, you reach the Safari Zone.
Go east and you reach a fence maze. The left part of the maze has Gentleman Walter, who has Manectric. Near the Gentleman, there is a Carbos.
To the east, there is Pokéfan Vanessa. She has Pikachu. You can cut the tree east of her to find a hidden Nugget.
To the south, there are berry trees.
Keep going east to reach Lilycove City.