After the game loads at the title screen there's a green "stickers" button, tap that and on the Stickers screen tap the top right button labeled "Enter #Tag". You don't need to type the # symbol, just the letters.

Code	Effect
cold	Unlocks bonus levels in the Frosty Forest.
cool	Unlocks Goldfish reward.
dive	Unlocks bonus levels in Bouncy Beach.
fast	Unlocks bonus levels in the Dusty Desert.
good	Unlocks bonus levels in the Frosty Forest.
grin	Unlocks bonus levels in the Dusty Desert.
jump	Unlocks bonus levels in the Dusty Desert.
kids	Unlocks bonus levels in the Grove of Goodness.
nice	Unlocks bonus levels in Bouncy Beach.
peel	Unlocks bonus levels in the Dusty Desert.
rays	Unlocks Seagull Reward.
sand	Unlocks bonus levels in Bouncy Beach.
sled	Unlocks bonus levels in the Frosty Forest.
snow	Unlocks Rainbow reward.
soar	Unlocks bonus levels in the Grove of Goodness.
spin	Unlocks Raccoon reward.
star	Unlocks bonus levels in the Grove of Goodness.
surf	Unlocks bonus levels in Bouncy Beach.
team	Unlocks bonus levels in the Grove of Goodness.
wish	Unlocks bonus levels in the Frosty Forest.