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