Well the first thing that came to my mind was to have a bunch of pictures that look like this
and next to it saying "Do you want our neighbourhood to look like this?".
Next I would get some chalk and draw fishes or turtles around the drains that would say something like "Stop feeding me trash". This idea actually came from my teacher because at the time we were learning about Water Pollution and we decided to draw fishes saying "Help" or " Respect the ocean ".
Lastly I would advertise about how important NZ is and that we have to keep it clean.