I just found out this cool thing! You know those places where footsteps appear in the sand as if something invisible was making them (The beginning of Deep Dark galaxy is an example)? Well don't move for a while, and just watch the footsteps, and this money bag will come out, but it's impossible to get it by running at it, shooting it, etc. Instead, use the second player to stun it by holding the A button with the cursor in front of it, then have the actual player attack it. Loads of star bits! (Although you might get coins if you jump on it to attack; I spun it to attack)
(By the way, if you know where I copypastaed this from, yes, that is my GameFAQs account. You can't blame me for that sucky name; I lakced creativity when I made the account)