Bonus life
Dig next to a flower to get an extra life.

Chicken hat in Level 1-4
Destroy the Benemoth building. You should hear a foghorn sound to indicate that the hat has been unlocked.

Extra lives in Level 3-4
After fighting the Boss (giant green robot with the eye and a fork and knife) and his life reaches zero, he will start exploding. Continue shooting at his eye. He will just continue exploding as long as you shoot, and you will continue gaining points. You can stop when you have about 60,000 points. At the end of the level, you will gain about seventeen lives.

Hats
Go into the options menu and enter one of the following names as a player name. Select "Submit", and you will hear the foghorn sound if you entered the code correctly. Go into the hat menu and the corresponding hat will be unlocked:

Super Soviet Missile Master mini-game
Successfully complete level 1-5 and the Super Soviet Missile Master mini-game will be unlocked after the intermission sequence.