After you go through Fortree City and find the Fortree Gym blocked by an invisible obstacle, you follow Steven to Route 120.
Explore the Route
To the east, you can battle Parasol Lady Clarissa, who has Goldeen and Seaking.
From there, you can go south down some stairs to find a Secret Spot and a Nest Ball.
If you Surf south from there, you can find Scorched Slab. You will explore this area later, but you can get some items here now if you want to. In the first room of Scorched Slab, you find TM11 Sunny Day. Go downstairs and go to the northwest corner to get a Full Heal. Use your Dowsing Machine in the northeast corner of this room to get an Escape Rope. Go downstairs and use the Dowsing Machine to find a hidden Nugget in the southeast part of the room. Then use the Dowsing Machine in the northeast part of the raised area in the middle of the room to find a hidden Super Potion. That is all that you can find in Scorched Slab for now, so just leave and go back to Route 120.
Go back up the stairs north of Scorched Slab. If you go northeast of Parasol Lady Clarissa, you find a tree that you can cut with Cut. Follow the path beyond that tree and cut another tree in the way. Use your Dowsing Machine at the end to find a hidden Rare Candy.
If you battled Gabby and Ty in Route 111 and Route 118, you can battle them here in the northeastern part of Route 120. They will now have Magneton and Loudred. After the battle, they will go back to Route 111. You can keep battling them in the three routes that you found them in. This gives you an infinite source of experience, as well as money, which you can improve by giving the Amulet Coin (from your Mom in Littleroot) to one of the pokémon that you send out against Gabby and Ty.
Go southeast from the first tree that can be cut with Cut. Use the Dowsing Machine to find a hidden Revive in the tall grass.
Go south through the tall grass and you can battle Bird Keeper Robert. He has a Swablu.
Go west from the Bird Keeper and cross the bridge. You will find Steven, who says that there is an invisible pokémon blocking the bridge. He asks if you are ready for battle. If so, he gives you the Devon Scope, allowing you to see the invisible Kecleon on the bridge. You will battle it, and you can either capture it or KO it. You can find Kecleon in other places, so it's up to you.
Afterward, Steven gives you the Devon Scope. He will also give you the Mega Stone that will allow your starter pokémon to Mega Evolve after it has been leveled up enough to evolve into its third form.
If you want to, you can go southwest from the bridge and battle Bird Keeper Colin, who has a level 32 Swellow and a level 32 Xatu.
If you try to go any farther, an Ace Trainer will stop you.
Now that you have the Devon Scope, you can reach the Fortree Gym, so go back to Fortree City and go down the middle ladder from the north bridge, then check on the invisible object and use the Devon Scope. The Kecleon will run away.
Before going into the gym, go behind it to find TM86 Grass Knot.
Then go into the Fortree Gym.