As with most Pokémon games, there are certain items that respawn after being obtained. These are all hidden items that you can only spot when your partner's tail starts wagging rapidly

External Hidden Items

Locations: Kanto

Within the various areas of the game you will find various hidden items, signified by your partner's tail wagging. Each day, the hidden items in the various routes will respawn giving you access to infinite berries, potions and more.

Caves - Daily

Locations: Celadon City Game Corner, Cerulean Cave, Mt. Moon, Rock Tunnel, Seafoam Islands, Underground Path 5-6, Underground Path 7-8, Victory Road, Viridian Forest

Within the various caves of the game, there are numerous hidden items. These typically appear inside rocks and include a myriad of items including the Ultra Balls and even, in Mt. Moon, the Moon Stone. These items are listed in their respective location sections so be sure to check the section to see the items

However, the items in the Celadon City Game Corner appear different each time you check them.

Cerulean Cave - Minutes

Locations: Cerulean Cave

On the top floor of Cerulean Cave is a floor filled with glowing sparkling spots. In these spots are a variety of items. This includes batches of Poké Balls, Great & Ultra Balls, even a Master Ball and Golden Berries. There is even a spot in the middle where the crystal offers Fossils and Max Revives

These items seem to change on each visit to Cerulean Cave 2F and each have a different time to respawn, with a certain number of steps, each item has got a chance to respawn. This is not a guarantee but walking about for a bit should typically respawn most of the items.