Find a patch of flowers and spray them all quickly. Four or five Coins will appear from the middle of the patch.
If you wash off a person with sludge, they may give you a Blue Coin.
Go on top of trees and you might find some extra Coins.
Spray the wanted of Mario posters in Delfino Plaza to get one Coin each. Some do not always give Coins.
Eat the green birds with Yoshi to get regular Coins. Eating blue birds will give you Blue Coins.
You can get Blue Coins for helping the people in Delfino Plaza get fruit and putting them into a basket.
Spray a symbol (X) with water until it goes away. A blue coin will appear from another symbol (and vice versa).
If you wander around in the manhole (usually in underwater tunnels) you will find yellow and blue coins.
Get coins in the harbor (with bloopers) by going onto the sky walks. There are tons of coins up there.
During the first level, when you approach a red circle on the beach, stomp there to find the question mark coins.