Eat a Notch Apple.

Use a saddle to ride a pig, then have the pig get hurt from fall damage while riding it.

Tame a Horse.

Tame an Ocelot.

Bribe a Dolphin.

Create an Iron Golem.

Dry a Sponge in a Furnace.

Defeat an Elder Guardian.

Gather all 16 colours of Wool.

Mine or purchase 30 Emeralds.

Use a Dried Kelp block as fuel in a furnace.

Dye your bed a different color.

Throw diamonds at another player.