All Characters in Multi-player Mode
Enter VRCLN as a password.

All Weapons
Enter the password NSRLS.

Cycle Through Weapons Backwards
To browse through your weapons in reverse order, During the game, hold the Z button and press the B button to cycle through each gun.

Debug Mode
To display some rather useless test information, enter the password THDTST.

Fry the Colonel
If your feeling particularly sadistic, feel free to slaughter your commander at either the obstacle course or the live fire range. He'll call the MPs out, but I've never actually seen them come.

Light the Candles
On the living room level when you get to the top of the couch, go near the ruler that is a bridge over to the other side. Use your flame thrower or your morter to light the candles.

Maximum Ammunition
Enter the password MMLVSRM.

Mini Mode
Enter the password DRVLLVSMM to become smaller.

Passwords

Level             Password
Attack            LNLGRMM
Spy Blue          TRGHTR
Bathroom          TDBWL
Riff Mission      MSTRMN
Forest            TLLTRS
Hoover Mission    SCRDCT
Thick Mission     STPDMN
Snow Mission      BLZZRD
Shrap Mission     SRFPNK
Fort Plastro      GNRLMN
Scorch  Mission   HTTTRT
Showdown          ZBTSRL
Sandbox           HTKTTN
Kitchen           PTSPNS
Living Room       HXMSTR
The Way Home      VRCLN

Play as General Plastro
Enter “PLSTRLVSVG” as a password.

Play as Vikki
Enter “GRNGRLRX” as a password.

Play as a Tin Soldier
Enter TNSLDRS as a password.

Restart Level
To restart the current level, hold L + R + down-C.