Here is the full list of all 13 Poly Party achievements.
Win a game of Tic Tac Toe in the arcade of Poly Mode!
Win a game of the basketball shooter in the arcade of Poly Mode!
Obtain the junkyard car that you used in the mechanic missions!
Purchase a house in the Poly Mode
Win a drag race in the Poly Mode
Win an event day in Racer Nation!
Deliver some mail for the mailman in Poly Mode!
Win your first minigame in party mode
Win your first boardgame in Party Mode
Beat a time trial in Racer Nation
Win a pinkslip race against a Boss in any Racer Nation event
Deliver a car through Online mode successfully.
Beat the high score on the video game Hurdle Jumper!