Great Game
Here's some help for those of you who are having trouble with the secrets:
First Level- Go all the way to the left and get the green gun. Blow up the left side of the house until you get the alien hominid secret (don't worry, you cant blow up the secret)
Second Level- Hurry up and out of the trees and go to the right to the gingerbread man. Blow him up. At the center of him will be the P-bot secret (grab it quick!)
Third Level- Right where you get the very first rockets in the level is where you need to be. shoot down until you dig through the level. Run to the right until you see picos picture in the ceiling. Blow it up and get it.
Fourth Level- I didn't find any secrets on this level, but I did find that the only way to kill the boss is to get him to blow a hole to the bottom of the level and he jumps in the water and dies.
P-bot has good health and a good up jump, but isnt good at jumping sideways. Pico has a quick shot and below average health. He's the best char for the boss level. Alien Hominid sucks (just like the real game!) and I didn't find the Goldfish char. Enjoy!