Different codes:
While in gameplay (During a campaign mission, Great battle, or special operation) hold Down on the D-Pad (Not the control stick) and press these buttons in sequence to get the desired effect.

X B X B Y Y - Para-troopers drop down in camera view (Only once a level)
Y X B A Y B - Gives 5000 plastic
Y B X A Y A - Gives electricity (!800 I think)
B B A Y A B - All of your units do extra damage and take less damage
Y Y A B Y Y - Makes enemies harder to defeat
Y B X A Y A - Gives 2000 Electricity
B B B Y X A - Dump Trucks Gather More Resources
B Y B B A X - Changes Army's Colour

Special Operations:
Midnight Snack (special operation 1) - Earn a gold medal in campaign mission 8

Lord of the Ants (special operation 2) - Earn a gold medal in campaign mission 9

Sandy Breach (special operation 3) - Earn a gold medal in campaign mission 10

This Old Fortress (special operation 4) - Earn a gold medal in campaign mission 11

Road Block (special operation 5) - Earn a gold medal in campaign mission 12

Tan Some Hide (special operation 6) - Earn a gold medal in campaign mission 13

Hostage Situation (special operation 7) - Earn a gold medal in campaign mission 14

Blastoff! (special operation 8) - Earn a gold medal in campaign mission 15

Great Battles:
There are 8 great battles. One is available from game start and the other seven must be unlocked.

Blind Pass (great battle 2) - Earn a gold medal in campaign mission 1
Basement Baloney (great battle 3) - Earn a gold medal in campaign mission 2
Bathroom Blitz (great battle 4) - Earn a gold medal in campaign mission 3
Flooded Follies (great battle 5) - Earn a gold medal in campaign mission 4
Tricycle Treachery (great battle 6) - Earn a gold medal in campaign mission 5
Waterway War (great battle 7) - Earn a gold medal in campaign mission 6
Sandbox Shenanigans (great battle 8) - Earn a gold medal in campaign mission 7

Alternate Menu Screen:
Alternate Menu Screen - Obtain all 31 gold medals.