Just as a puppy is for life, not for Christmas, I think some of the ways we observe World Environment Day should be life choices rather than simply leaving the car at home for the day (and then cuddling up to the convection heater at work, drenched and grumpy, and undoing all that good karma). And the best strategy I can think of in this regard is - snip!
Child-free people are often accused of being selfish, but I've yet to hear an argument for having children that wasn't fundamentally selfish. Of course, in the absence of safe, effective and acceptable contraception, accidents happen, but luckily these days abortion on demand is available as a fix.
Each child created puts additional strain on the planet. And the wealthier you are, the greater the strain. The child of a subsistence farmer will cost the planet less in sustainability than the pampered darlings of Britney Spears or - please let this never happen - Paris Hilton. Resource consumption, disposal of waste, the size of their ecological footprint...
Love the planet a little. Spread the love, but keep your genes to yourself.