After you make your way through the Reflection Cave, you arrive in Shalour City, where you should look for the Mega Evolution guru.
In the Pokémon Center, you find Mr. Bonding. He gives you Critical Power.
In the house east of the Pokémon Center, the researcher will give you an Eviolite if you have enough pokémon in your Coastal Kalos Pokédex. The Eviolite raises Defense and Sp. Def, but only if the pokémon holding it has not reached its final evolutionary form.
West of the Pokémon Center, there is a house where the old woman will give you a Soothe Bell if your lead pokémon likes you.
In the house west of there, the guy near the door will ask to see one of your pokémon and will give you a ribbon depending on the relationship between you and that pokémon. In the same house, the girl in the corner will give you Stardusts if you have a pokémon with you that is Flying or Psychic type.
If you go east of the Pokémon Center, you reach Route 12, but there's a guy who won't let you pass until you win the gym badge from Korrina. There is some fenced-in grass to the north, and you can find a hidden Honey in the northeast corner of the fence.
In the grass of route 12, you can find Miltank, Tauros, Chatot, Heracross (Y only), Pinsir (X only), Slowpoke, Pachirisu, Exeggcute. You may also encounter hordes of Mareep, Tauros, Miltank, and Wingull. Go south through the grass here and use Cut on the prickly tree to find an Aspear Berry tree.
If you go north from the Shalour City Pokémon Center, Tierno and Trevor come up and mention that the Mega Evolution guru is in the Tower of Mastery, which is at the center of the city.
In the house northwest of the Pokémon Center, there is a psychic who will trade berries with you.
If you go north to the sandy area, Tierno comes up to you and gives you an Intriguing Stone. The two of you look to the north and see the Tower of Mastery.
To the north, there is a photo spot.
West of the photo spot, you can use your Dowsing Machine to find a Stardust on a rock.
Now go north along the path toward the tower. Before you go under the archway in the brick wall, go east and get the Max Ether.
Now go north up the stairs. Before you go into the tower, use your Dowsing Machine to find a Max Repel on the bushes to the right of the entrance. Then go into the Tower of Mastery.