Successfully complete one of the following tasks to get a trophy:

The Prince (Silver): Take the Prince for a ride.

Additionally, there are eleven secret trophies:

10 (Bronze): Take 10 inhabitants for a ride.
20 (Bronze): Eat 20 inhabitants.
100 (Bronze): Stretch BOY to a length of 100m.
Bye-Bye BOY (Bronze): Meet up with Bye-Bye BOY.
The BOY House (Bronze): Rotate the roof of the BOY House.
The Space Squirrel (Bronze): Find the Space Squirrel.
BOY Quiz Expert (Bronze): Answer all questions correctly in the BOY Quiz.
765 (Silver): Report a length of exactly 765m to GIRL.
Unproportional (Silver): Make BOY's body grow thicker while keeping his head small.
Combined Character (Silver): Create a combined character by eating an animal and a human or a fruit and a human.
Eat Your Words! (Gold): Eat the words NOBY NOBY BOY within the manual or in the staff credits.