Here’s what can happen when a pinata is destroyed.
Spawns a cow.
Spawns a donkey
Spaws a Bee
Spawns a Chicken
Spawns a Rabbit
Spawns a creeper
Spawn a wolf
Spawn a horse
Spawn Skeleton horse
Spawn Slime
Spawn Boat
Spawn Ghast
Spawn Mooshroom
Spawn Giant Slime
Spawn Trader
Lightning strike
Spawn a Charged Creeper
Spawn a cat
Spawn Skeletons
Rain Fish
Snowday - Adds a layer of snow 
FrogRain - Rains Frogs
Spawns a few birds and a music disk “minecraft:music_disc_chirp”
GravityShift - Give all entities levitate for a few sec.
Give Strength for a few min.
Give Nausea for a few sec.
Give haste for a few min.
Give Speed for a sec.
Give Nightvision for a few min.
Give Blindness for a few sec.
Give Fire resistance for a few min.
Give Jump boost for a few min.
Give Absorption for a few min.
Change the day to morning.
Change the weather to clear.
Change the weather to thunderstorms.
Birthday party - spawn a cake and play a custom birthday tune. 
Drops a Confetti Sword - When hitting something with the sword, confetti spawns. It deals 6 damage per hit
Drops a rainbow armor - Spawns a rainbow trail after the player.
Spawn shark particles
Drop a Drum instrument
Drop a Kazoo instrument
Drop a Trumpet instrument
Rocket Launch - launches the player up as a rocket.  
Drop a knockback glove - a weapon that deals 0 damage but has a strong knockback.
Drops a stack of pinatas.
Drops a hot potato which damages the player when held.
Drops a special cookie which randomly teleports the player when eaten.
Give the player a custom wither aura for a few min.
Spins the player for a few seconds.
Give the player a custom knockback aura for a few min.
Spawns a party pig - Custom pig with a party hat.
Spawns a flying pig.
Gives a fish weapon which makes a sound when something is hit. deals 0 damage.
Spawns a friendly creeper.
Spawn a rainbow sheep.
Spawn a Dinosaur.
Spawn a custom party dog.
Drop butterfly wings - chestplate which gives “Levitation” when worn.
Spawn candy particles.
Spawn Cat particles.
Spawn banana particles.
Spawn Chicken particles.
Spawn Dinosaur Parts particles.
Spawn Lollipop Parts particles.
Drop a rainbow sword - When hitting something with the sword a rainbow appears and deals damage. Sword deals 4 damage, rainbow deals 3 damage.