At 8 PM, we read an Earth Day story, then went online to learn about Earth Hour, did some work papers (colouring sheets, word searches, etc.), then the girls ran around the house unplugging stuff and turning off lights.
By candle light we did puzzles (not as hard as you'd think) and played board games. The girls had a blast wearing glow-in-the-dark bracelets and waving their glow-in-the-dark wands (I'm playing dumb about the environmental unfriendliness of that stuff). Definitely a day to remember.