Here is the full list of all 38 Pile Up! Box by Box achievements.
Watch a performance by colorful friends
Strike a pose on a pedestal
Open the bonus route in Feather Theater
Bounce a trampoline... on another trampoline
Watch a fireworks show with spiky friends
Do something horrible...
Open the bonus route in Forest Fair
Explore all of Boxling Square
Watch a movie with upside-down friends
Build an autonomous flying box
Fall to your doom 100 times
Put on a new outfit
Listen to a musical performance by some slimy friends
Open the bonus route in Frogipelago
Long jump: dash and jump!
High jump: heavy dash and jump!
Carry an item through 10 consecutive rooms
Finish the tutorial (no skipping)
Make a pile of 5 boxes or more
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
Play every game in the arcade
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
Complete Frogipelago 100%
Open every beach umbrella
Open the bonus route in Citizen Cave
Smash every squash in the game at least once
Complete Citizen Cave 100%
Move the wall of boxes in the cave to another room
Complete Forest Fair 100%
Complete Feather Theater 100%
Unlock all other achievements