Here is the full list of all 38 Pile Up! Box by Box achievements.
Watch a movie with upside-down friends
Long jump: dash and jump!
Build an autonomous flying box
Fall to your doom 100 times
Open the bonus route in Citizen Cave
Put on a new outfit
Listen to a musical performance by some slimy friends
Make a pile of 5 boxes or more
Finish the tutorial (no skipping)
Explore all of Boxling Square
Pick up every collectible in the game
Make a pile of 10 boxes or more
Make a very big pile of 20 boxes or more
Throw a pile of 10 or more boxes into the water
Carry an item through 10 consecutive rooms
Play every game in the arcade
High jump: heavy dash and jump!
Destroy a pile of at least 10 boxes
Finish the game faster than the Seed By Seed record
Buy every costume
Scare every spider in the game
Complete a world without falling
Strike a pose on a pedestal
Open the bonus route in Frogipelago
Complete Frogipelago 100%
Open every beach umbrella
Smash every squash in the game at least once
Complete Citizen Cave 100%
Watch a fireworks show with spiky friends
Open the bonus route in Forest Fair
Move the wall of boxes in the cave to another room
Complete Forest Fair 100%
Watch a performance by colorful friends
Open the bonus route in Feather Theater
Complete Feather Theater 100%
Do something horrible...
Bounce a trampoline... on another trampoline
Unlock all other achievements