Coins drop from enemies that die by your towers. The coins don't last long out on the field, so make sure to grab any, and watch the enemies as they get attacked by towers--so that you can see if any coins fling out and hide behind trees.
The good thing about coins is that they "magnetize" to your Tiki Man, so if you're near a coin that's sitting in the middle of a wave of Monsters, it'll still gravitate to you. Coins usually like to orbit Tiki Man, so you still have to take the last step to grab them.
Also to note, if you touch a Monster, you will crash and many coins will fall out and spread around you. You can try recovering those, but usually you end up losing some. Stay away from enemies and just learn the acceptable distance before money begins to gravitate towards you if there are enemies near your coins.
I'd suggest doing this on a level with narrow paths, so that the coins usually focus to being tossed on that path, or the immediate trees around it. Also, there are usually 3 sets of "hidden coin batches" within the trees on each map (as well as 1 hidden gem tree, sometimes mixed with coins or by itself), so if you touch a tree and coins spew out, [B]GOTTA CATCH 'EM ALL[/B]