Our building recently planted trees along the sidewalk (full disclosure, my harder working neighbors are the ones to thank, I was away). Planting trees, which is the activity Arbor Day is centered around removes CO2 from the air, produces oxygen, beautifies the city, and helps to fight global warming.
To get involved and plant a tree yourself, take a look at this week's website A Million Trees NYC.
The Boggias, who run this place, are wonderful people.