In the Playground Peril level in tournament mode, shoot a missile at the middle swing on the swing set that is in the background next to the merry-go-round. A small Axel icon will be thrown into the playing area. Pick it up to unlock Axel.
Buster's Lanes level
Get twenty kills in an endurance match.
Darkside
In the Easy Death Oven level in tournament mode, go to the sink and drive down the plumber's butt. Go onto his leg and napalm his butt. He will blow a hole in the wall. Enter the hole to find the icon that unlocks Darkside.
God mode
While playing, press right + square, left + circle, right + square, left + circle, right + square, right + square, right + square, right + square.
Holiday Havoc level
Get thirty kills in an endurance match.
Infinite weapons
While playing, press left + cross, down + square, right + triangle, up + circle, left + cross, down + square, right + triangle, up + circle.
Invulnerability
While playing, press right + cross, right + cross, left + cross, left + cross, up + circle, up + circle, down + circle, down + circle.
Killer weapons
While playing, press up + triangle, up + triangle, left + square, left + square, down + square, down + square, right + triangle, right + triangle.
Mime
In the Holiday Havoc level in the medium difficulty setting in endurance mode, shoot missiles at the Christmas tree until it is destroyed. Get on top of the fireplace then jump off backwards and shoot a missile at the stockings. Destroy all four to reveal the icon that unlocks Mime.
Piecemeal
Successfully complete the game as Trapper to unlock Piecemeal as a playable character.
Playground Peril: Hidden health
Shoot the big duck to get a health power-up.
Playground Peril: Shoot boy on merry-go-round
You can shoot missiles at the boy on the merry-go-round from the playhouse. He will spin so fast that he will fly off.
Successfully complete the game as Mime to unlock Trapper as a playable character.
Tree house: Lawnmower
Shoot the lawn mower and it will hurt your opponents. If you have the skill, get behind it and shoot, then turn around and go to the switch the lawn mower will fall in the hole and break.