Creeper Farm (Gunpowder)

The trick to a good creeper farm is limiting headroom so only creepers can spawn there, not zombies or skeletons competing for the same slots. Open trapdoors at head height across the whole platform leave a 1.5-block gap, tall enough for a creeper, too short for most other hostiles. That's why nearly every creeper farm you'll see has a literal ceiling made of trapdoors.

Building over ocean instead of land also helps, since the game's mob cap is shared across everything nearby. Fewer spots for drowned, zombies, and skeletons to spawn near your platform means more of that cap goes to creepers instead.

A cat parked near the drop point isn't decoration. Creepers are actually scared of cats and will flee from one, so putting a cat at the far end herds them toward the fall or the funnel instead of leaving them to wander the platform at random. Noticeably faster collection.

For the kill, most builds either use a long fall onto a hopper floor for full AFK automation, or a shorter fall plus a manual finishing hit with Looting III, which roughly doubles gunpowder per kill if you're willing to actually stand there and swing.

A working farm produces gunpowder faster than almost anything else uses it up, except maybe constant TNT. It's the bottleneck ingredient for fireworks (elytra flight depends on this), splash and lingering potions, and TNT itself, so it's usually worth building even in a world where you're not fighting many creepers otherwise.