Name a garden Piñata People (must have the special character 'n').

After that you don't have to do any thing to the garden (you do this to unlock credits instead).

Go into the credits and smash all the Pinatas. This will unlock a well (note: you can't do this in 'just for fun' mode).

Buy the well and throw in 1,100 chocolate coins and a lot of coins should pop out of the well.