I created this little toy in Processing called GardenGun
http://kreitmayer.com/gardengun
(may take a few seconds to load)

Some people have found it quite addictive and said they would love to play it on their Android devices. I personally don't have an Android but if any of you would be keen to port GardenGun to this platform and put it on the Market, please email me: stefankreitmayer[a t]gmail(d o t)com