After cycling through the full credits, if you completely cycle through them again you will hear the "coin noise" and will be rewarded with 50 coins. This can be done multiple times. (I don't believe there is a cap)
12/13/2004
New Items
You can change the system date on the DS to a different time or date in order to get new items in the shop. This works well for going back to a date that you missed, or forward to a date that you cannot wait for (like Christmas).
12/17/2004
Shantae background
Touch the Ping Pals logo exactly at midnight (00:00:00) when the chime sounds to unlock Shantae's background in the Backgrounds section of the Items Shop. TIP: Try setting the system clock fifteen seconds before midnight to give you enough time to prepare to do this.
1/4/2005
Save Coins on Hi-Lo
When you lose in Hi-Lo (get more than 4 off), if you quit the game, by going to the shop, starting a new minigame, or saving and quitting, before Snakey takes away your coins, you won't lose the coins you would have lost.
4/11/2005
Holiday Items
On certain days of the year, special items can be bought. Play the game on that date to have access to the items.