GS codes that access stuff that was left in, but isn't accessible in the normal game. The codes below are ones I've tested so far.
Note: All GS Codes are from Gameshark Central and the Rare Witch Project.

Image Modifier:

Banjo	812986BA xxxx 812986BE xxxx
02F5	Green BK Flag Pole. I don't remember this being in the game. Image
02F9	Red BK Flag Pole. This might have been in the game, I'm not sure.
Talking 2D Head Codes - Conga	8136C9D8 xxxx
06DC	Christmas Tree. Did this talk? Can't remember. Image Image
0811	Beta Empty Honeycomb. Looks slightly different. Image Image
082F	Tip Tup Choir Turtle. NOT Tip Tup! This certainly didn't talk. Image

Music Modifier:

Music Modifier	8128189A xxxx
0039	Advent - Fungus Forest track
004D	Mumbo's Raindance - beta Mumbo's Mountain track?