Cheat mode
Enter 3 Groundhogs (found towards the end of the name entry list of characters) as a name to unlock an extra "Secret Garden" menu at the options screen. This also happens after 44 levels. The extra options are:
Riki's shot trail thickness - Normal (thin), Thick
Mami's shot trails - Normal (thin), Thick, Large Missile and Thick (Nintendo 64 style)
Explosion Fragments - Normal, None, Heavy
Background - Normal, None, ?
Invincibility - Off