To get lots of coins, go to 5-2 and fly to the upper area of the first screen where two groups of Green Koopas walk back and forth between pipes. Stomp one and kick the shell to knock out all the others, producing lots of big coins. Since they instantly respawn, you can keep going back and forth between the two groups. Alternatively, if you have the Courage Soda, just hit them all with your umbrella instead—you'll get even more coins that way.