Level skip - Hold RT to kill yourself repeatedly until the "Coward's Way Out" option appears, which allows you to skip levels.

Scientist Gamerpic - Successfully complete Level 1-4 in Multiplayer mode to unlock the Scientist Gamerpic.

'Splosion Man Gamerpic - Successfully complete Level 1-4 in Single Player mode to unlock the 'Splosion Man Gamerpic.

'Splosion Man Premium Theme - Successfully complete Level 3-18 in Single Player mode to unlock the 'Splosion Man Premium Theme.

Dashboard Avatar t-shirt - Break any glass to unlock the 'Splosion Man t-shirt.

Doughnut song - During the first chapter, hold one of the fat scientists and stand still.

Achievements - Accomplish the indicated achievement to get the corresponding number of Gamerscore points:

Get Them Out of Our Schools (10 points): Eliminate 10 Scientists and stop them from spreading their filthy lies.
Master of Controls (10 points): Master the mastery of controlling your controls (edit the controller configuration).
You're the Best Around! (25 points): Complete the Single Player game.
A Little Help from Your Friends (25 points): Complete the Multiplayer game.
Omaha Steaks, Delicious! (15 points): Create 5,000 filets.
Not a Portal Reference (25 points): Collect all 47 cakes in Single Player mode.
Sugar High (25 points): Collect all 47 cakes in Multiplayer mode.
Get Over Yourselves (10 points): Select "Credits" from the "Help & Options" menu and watch the whole thing.
Only Gandhi Would Be Proud (10 points): How long can you handle being a pacifist?
Lay Off the Caffeine (10 points): Let your hyper little brother play the game for a bit.
All Hail The Splode King (25 points): Complete the Single Player game in Hardcore mode.
Eventual Destruction of a Galaxy (10 points): Be on the lookout for an old friend.