When you kill zombies they will sometimes drop keys as well as spare change. Applying Plant Food to Sunflowers will give your Sun currency a huge boost and applying it to offensive plants will enable them to inflict a lot of damage.
Remember though that you can only hold three doses of Plant Food in the bank at any one time unless you upgrade this bank limit. You can also spend game coins to buy more plant food if you have run out. Kill glowing zombies and add the food they drop to your bank by touching it. If you still need coins and you have real world money to spend there is always the option of the in-app store. When you complete levels without falling back on your lawnmower defenses you will earn bonus coins for every lawnmower that you have remaining at the end of the level. You can always replay the levels that offer the bonus coins to get more cash. Zombies will drop spare change whenever you kill them and certain levels will offer you bonus coins. Instead of using all your power-ups in the regular levels save them for the Brain Busters where the right power-up will enable you to beat the tricky levels without any problems and get the stars with ease. When you complete the final 'day' of each world you can return to levels that you have already beaten and earn more stars. Complete the levels known as Brain Busters to earn stars to unlock more worlds.