After fighting the Team Magma/Aqua admin in the Weather Institute of Route 119 and battling your neighbor, you go east to Fortree City.
Visit the Secret Base Guild
The southeast tree house in Fortree City is where you can find the Secret Base Guild. At the guild, you can get items for your Secret Base.
Talk to Aarune and ask him to look at the flags that you have collected. If you collect a lot of flags from other players' Secret Bases, your rank will go up and you will get special rewards. The prize for getting the highest rank includes a Mega Stone: Garchompite. You can also battle Aarune once per day.
Talk to the person in the upper-right of the room to change the type of trainer that you will look like when others visit your Secret Base.
Use your Dowsing Machine to find a hidden Super Repel to the left of the Poké Mart.
In the northwest house, you can trade a Spinda to get a Skitty named Skitit.
In the house east of that, talk to the woman on the left to play a guessing game. If you guess correctly, you get TM10 Hidden Power. The answers to the guessing game are always the same, so you can just keep trying until you get it. But if you don't want to bother with that, the answers are here. Click the box to show them: right, right, left.
Hidden Power is listed as a Normal-type move, but the move's type is actually determined by a pokémon's hidden stats. The other old woman in the Hidden Power house can tell you what type of move Hidden Power will be for a particular pokémon.
If you try to go down the ladder in front of the next house, you will be blocked by something invisible, so you won't be able to go to the Gym right now.
In the house east of that one, talk to the Pokémon Breeder and his Wingull will leave. Come back to this house after you get the next Gym Badge.
Go down the ladder from one of the easternmost tree houses. Go toward the east and you will see Steven, who goes east to look for a pokémon in Route 120.
Before following Steven, use your Dowsing Machine in this eastern area to find a Tiny Mushroom near the Pokémon Ranger.
Then go east to Route 120.