Enter these from the Mystery Gift option at the title screen, via the password distribution option. They will give you gifts in the game! Note that Mystery Gift passwords require an internet connection, and may expire after a specific time.

Password	Effect
AZUL	Mega Stones - Pidgeotite, Houndoominite, Heracronite, and Steelixite.
DRACHE	Mega Stones - Ampharosite, Altarianite, Latiasite, Latiosite.
INTIMIDATE	Mega Stones - Mawilite, Beedrillite, Audinite, & Medichamite.
M2DESCENT	Mega Stones - Mewtwonite X & Mewtwonite Y.
MATSUBUSA	Mega Stones - Sceptilite, Blazikenite, Swampertite, Cameruptite, and Banettite.
PIKACHU20	Pikachu wearing Ash's Alola Cap from October 24-30, it comes with Pikashunium Z-Crystal.
PIKACHU20	Pikachu wearing Ash's Hoenn Cap from October 24-30, it comes with Pikashunium Z-Crystal.
PIKACHU20	Pikachu wearing Ash's Kalos Cap from October 24-30, it comes with Pikashunium Z-Crystal.
PIKACHU20	Pikachu wearing Ash's Original Cap from October 24-30, it comes with Pikashunium Z-Crystal.
PIKACHU20	Pikachu wearing Ash's Sinnoh Cap from October 24-30, it comes with Pikashunium Z-Crystal.
PIKACHU20	Pikachu wearing Ash's Unova Cap from October 24-30, it comes with Pikashunium Z-Crystal.
PIKACHUM20	Pikachu wearing Ash's Original Cap, it comes with Pikashunium Z-Crystal. Can be obtained even if you received one of the other special Pikachu before.
POYONG	Mega Stones - Gardevoirite, Galladite, Lopunnite, and Diancite.
SABLEVOLANT	Mega Stones - Tyranitarite, Abomasite, Manectite, and Aggronite.