Throughout Hyrule are temples acting as trials for Link to conquer and acquire new items to aid him in his quest.
In these structures navigation requires usage of the hero's abilities, the items he acquires and keys to unlock doors.
In the overworld there are a few areas that feature a small key one being held by Coro, another in a cavern inside Faron Woods, a couple being held onto by King Bulblin, and the final one held by a Bulblin guarding the gate leading to Arbiters Ground featured in the Bulblin Camp.
There are a couple dungeons that feature unique keys in addition to the pre-existing ones to add onto the complexity of these dungeons.
Snowpeak Ruins features a unique set of additional items that act as psuedo keys for the dungeon in addition to the standard keys which are the Ordon Pumpkin and Ordon Goat Chese respectively. This dungeon also features a uniquely designed big key being the Bedroom Key which was used to lock the room were a missing piece of the Mirror of Twilight rests.