Swim in lava while having the Fire Resistance effect.

Perform a perfect combo including all of Batman's combat moves (any play mode)

Throw diamonds at another player.

Brew a potion.

Play a music disc in a Jukebox.

Relieve a Blaze of its rod.

Build some bookshelves to improve your enchantment table.

Create an Iron Golem.

Construct a Dispenser.

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

Become a Spectre

Gather all 16 colours of wool.

Achieve 8 medals on combat challenges