Amity Square is a park north of Hearthome City where you can walk around with your pokémon, cook poffins, and find items.
Entering Amity Square
Go northeast or northwest from Hearthome City to find an entrance to Amity Square. You can only take a cute pokémon, such as your starter pokémon.
Interacting with Your Pokémon in Amity Square
The other eligible pokémon in your party will also be in the park, and you can talk to them to get them to follow you.
Talk to your pokémon and it may sometimes give you a berry.
If Your Pokémon is Blocking You
If your pokémon is blocking your path, just walk toward your pokémon and it will temporarily go back into its pokéball, allowing you to go past where it was.
In the southern part of the park, there is a TM43 Volt Switch.
Northeast of that, there is a TM45 Attract.
Around the ruins in the northeast of the park, you can find a Spooky Plate and an Amulet Coin.
Talk to the person at the blender and you can cook Poffins while your pokémon watch.
Unlock Walking Together with Pokémon
When you leave Amity Square for the first time, you unlock the ability to let your pokémon out of the pokéball to walk around with you. You can do this after you leave Amity Square. Just select a pokémon from your party and choose Walk Together. You can talk to the pokémon who is walking with you, for a chance to get a berry.