After completing a streak of some number of wins at the Battle Tree, the next day on your return you will receive an item from the receptionist depending on how long your streak was.

Unlockable	How to Unlock
Ability Capsule	50-win streak.
Bottle Cap	30-win streak.
Lansat Berry	100-win streak.
Moomoo Milk	5-win streak.
PP Max	40-win streak.
PP Up	10-win streak.
Rare Candy	20-win streak.
Starf Berry	200-win streak.