Holiday Outfits:
Change your system's date to either December 24th or 25th to see the turtles in Santa garb. Change it to October 31st to wear a pumpkin on your head.

Password List:
Enter these codes in the Password screen of the Options menu.
Alternate Costume (Raphael): SMLSR
Alternate Costume (Leonardo): MSRLR
Alternate Costume (Michaelangelo): MMMSL
Alternate Costume (Donatello): LMSSL
Attack Power Up (Raphael): RDSRL
Attack Power Up (Leonardo): LMLSD
Attack Power Up (Donatello): DRLDS
Defense Doubles (Leonardo): SSRSL
Defense Doubles (Donatello): LRLDD
Defense Doubles (Michaelangelo): LSSDR
Defense Doubles (Raphael): DSRRS
Defense Power Up (Michaelangelo): MSRMM
Change Sound Effects: SLSRS
Power Up Effects Double (Raphael): DLDRS/LMMDR
Power Up Effects Double (Donatello): SDDLS
Power Up Effects Double (Michaelangelo): LMLMS
Power Up Effects Double (Leonardo): LDDRS
Unlimited Fire Shurikens/All Food is Pizza (Michaelangelo): MDMSL
Unlimited Shurikens (Leonardo): SRRDM
Unlock Playmates Toy Data Base: LSDRM
Unlock Splinter for Story Mode: LDRSM

Unleash the Secret of Gembu:
Beat the third Dojo Stage with all four turtles to learn the Secret of Gembu maneuever. Use the move to damage everyone on the screen. You'll have three opportunities per level to use it.

Unlock Challenge Mode:
To unlock Challenge Mode, defeat Story Mode with all four turtles.

Unlock Characters For Vs. Mode:
Fulfill the following criteria to unlock the corresponding characters for Vs. Mode:
Casey Jones: Beat Stage 1 with Raphael
Evil Turtlebot: Beat Stage 3 with any turtle
Hun: Beat Stage 6 with Michaelangelo
Splinter: Beat Dojo Stage 3 with Leonardo
Shredder: Beat Shredder with any turtle
Oroku Saki: Beat Saki with any turtle
Yoshi Hamato: Beat Challenge Mode with any turtle

Unlock Levels for Story Mode Stage Select:
Complete a level to unlock it in Stage Select.