Chests are the biggest coin source in the game. There's 5 different kinds of chests(kind of):
Wooden chests-give minimum 5, maximum 20 coins.
Golden chests-give minimum 10, maximum 40 coins.
Fake chests-give 16 coins.For more information about this enemy please click here.
Stone chests-give 100 coins. And is the only chest not guarded by anyone or anything. It's a really rare chest.
Lottery chests-give 150 coins. Its a really cool chest, even though it looks like a normal golden chest it is the biggest coin source in the game