Get a gold medal in each activity:
20,000 leaks - Plug the leaks as fast as possible. Leaks connected have to be plugged at the same time. You can use your head and shoulders too!-EASIEST
River Rush - Take it slow and collect the pins. Stay on the banks or clouds for an easy gold-MEDIUM
Space Pop - Pop as many bubbles as possible-EASY
Rally Ball - Spread out as much as possible and hit the ball as hard as possible. Be quick!-EASY
Reflex Ridge - Speed is important. Jump for speed whenever possible, and dodge the obstacles-MEDIUM/HARD
You MUST unlock this in free play.
Good Luck!