Eiffel Tower in Paris, Sydney Opera House, Millennium Stadium in Cardiff, Edinburgh Castle and many other landmarks will switch off their lights. Also, London Eye will dim their lights for an hour as well.
Kuala Lumpur skyline, before and after lights off for an hour.
My uni is participating in this activity though it's not known among every student about this.
Although lights off in many places, we still have moonlight and you can have a picnic out there of having candlelight dinner. Ain't that good?
Anyway, HAPPY EARTH DAY!