Various Cheats
Enter any of the following "Passwords" as a password:

DNSTHMN = Level Select
NSRLS = Weapons
MMRTL = Invincibility
MMLVSRM = Max Ammo
CNTN = Unlimited Continues
DNLVSHSF = Invisibility
THDTST = Test Info
LVNGLRG = Living Large
DRVLLVSMM = Mini Mode
TNSLDRS = Tin Soldier
BGGRN = Big Green One
GRNGRLRX = Vikki
PLSTRLVSVG = Plastro
SHRMNSLDR = Hail Mendheimicus

Level Passwords
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

Reverse weapon selection
Hold Z and press B during game play to cycle through weapons in reverse order.

Forest level fires
Get a bazooka, then find a location in the forest where there is a small gap between some trees. Shoot straight, without hitting a tree. Three or four trees will ignite even though the shot missed them.