In the level where the door to the brown cave is, use a bomb-jump to jump over the door to the very right, then the coin will appear on the very left...
then as joshua said, there is a coin in the 1st level of the brown cave.
Also, there is a coin in the level with the 2 tomatoes (the last level with enemies in the brown cave), and to get it you have to put yourself where the right tomato is in the begining, and jump - the coin will appear right under you...
and in the level after it where there are no enemies, but 2 ledges (on the left and on right) you jump from the left one to the right one (a pretty difficult bomb-jump!) and there will appear a coin...
And finally - the 2nd coin that joshua mentioned, in the last level of the brown cave, just put a bomb on the very left of the level and the coin will appear!
Hope this helps!
