(Import)

Unlock the Cheat Menu - Enter three groundhogs as your name (it's one of the char's at the end of the alphabet)Get to the Extra Options menu when backing out of the Options screen (using the B key or returning using the menu) The options are:
Riki's shot trail thickness:
Normal (thin)
Thick

Mami's shot trails:
Normal (thin)

Think
Large Missile and Thick (N64 style)

Explosion Fragments:
Normal
None
Heavy (cool!)

Background:
Normal
None
?

Invincibility:
Off
On

The cheat also opens up 44 levels.

Gameshark codes

[M] Must Be First 9C5D88F8
[M] Must Be Last 245EECA9
Max Points 87CDDF2C05F5E0FF
Infinite Time 249DC4140000012C
Super Explosion Count 872C5F2C000003E7
Enable All Levels BF3FC9A80000002C
All Levels Complete ED386DEB0000002C