Every time you defeat most bosses in Career Day, you will receive either a high-value regular reward, or a Key Item. Key Items unlock various areas of the game, which usually include Boss locations that can have more Key Items.

Bosses that do not award a Key Item include all bosses in The Cleft of Dimensions, the bosses that replace the four Gargoyle statues outside the sealed dungeons in World 3, and Exdeath in his Castle in World 2.

Key Items List

Four Tablets: For each tablet you have, you can progress further into the Void. You will need all four to complete Career Day. Each one also allows you to pick three rewards from the basement of the Sealed Castle as usual; if reward tiering is on, these will be Tier 9 rewards!
Adamantite: When landing the airship in W1, you can now choose to fly ‘up’ to the Lonka Ruins for two boss fight locations. In addition, if you were locked to W1, you can now use the WarpShard to travel to W2.
Anti-Barrier: Removes the invisible barrier around Exdeath’s Castle in W2, leading to a single boss location in the basement. With the Bracelet, also allows access to the real Castle for further bosses. If W3 is locked, this item is mandatory.
Big Bridge Key: Allows you to enter the Big Bridge in W2, for a single boss fight.
Bracelet: Allows you to enter the real Exdeath’s Castle in W2, assuming you also have the Anti-Barrier. This unlocks two boss locations, as well as the opportunity to fight Exdeath, which may be mandatory if W3 is locked, or is optional otherwise.
Elder Branch: Allows you to enter the Great Forest of Moore in W2. This leads to a boss location, after which Cara will replace Galuf. Having Cara is also the prerequisite to entering the Fork Tower in World 3, granting two more boss fight opportunities.
Hiryuu Call: Causes the Hiryuu to appear in W2, next to your default location. This allows you to land on Xezat’s ship and fight a boss there.
Ifrit’s Fire: Allows you to pass a bookshelf in the Ancient Library in W1, to reach the second boss location. The first boss of the Library is not locked behind this item.
Mirage Radar: Allows you to find and enter Mirage in W3, in the tiny forest on the very south of the map. There are two hint locations to be found here, as well as eight unique shops; with reward tiering enabled these will be very high value shops. The Black Chocobo can be found here as well, which will allow you to reach North Mountain in W3 and fight the boss at Bahamut's location.
Moogle Suit: Allows you to enter the Moogle Village in W2. This only contains some free items; with reward tiering enabled, these will be fairly high tier items.
Pages (Pyramid Page, Shrine Page, Falls Page, Trench Page): These key items allow you to open the doors at the start of the Pyramid, Solitary Island Shrine, Istory Falls, and Great Undersea Trench dungeons in W3. The Pyramid and Shrine have a single boss at the end. The Falls requires the Submarine Key as well, and has a boss. The Trench also requires the sub key and has a boss, but also has two unique shop locations and three hints within.
Sandworm Bait: Allows you to fight a boss in the Desert of Shifting Sands in W1.
Steamship Key: Allows you to enter the Steamship, which is marked by a shrine outside Karnak. This leads to a boss location, then immediately to the Karnak Escape sequence, for another boss location.
Submarine Key: Causes the Submarine to appear in W2, next to your default location, and also allows the airship in W3 to submerge. In W2, this allows you to enter the Barrier Tower for a boss fight. In W3, this allows you to enter Sunken Walse Tower for a boss fight. With the Falls or Trench pages, you can also access those locations for additional boss fights.
Walse Tower Key: Allows you to enter Walse Tower in World 1 for a single boss fight.

Exdeath (Castle) Rewards

If you are playing with World 3 locked off, defeating Exdeath in his castle will allow you to use the WarpShard to enter World 3.

If you are playing without World 3 access locked, defeating Exdeath will grant you two different rewards. First, Tree Exdeath at the start of the final battle will appear with only 1 HP and modified AI. Second, you will be granted a choice among three duplicate key items. These items will never be mandatory to clear the seed - they are purely a bonus.