What's a cooped-up couple to do? Copulate, of course.
Pretend electricity is not an option and you're unable to carry on with your usual evening routine. Then abide by all of the urban myths that suggest a baby boom occurs nine months after every blackout and blizzard, and get it on. Since you can plan ahead here, remember to use protection if you're not looking to procreate.
Enjoy a romantic, candlelit dinner
Light some candles, pop the cork, and enjoy how good your mate looks illuminated by the soft glow. With no electricity, dinner will either need to be prepped and ready in advance of Earth Hour, or you could take the noncooked route. Think foods that are reputed aphrodisiacs: asparagus, almonds, figs, oysters, and chocolate for dessert. After sharing this spread, you'll want to keep the lights out for longer.
Gaze at the stars
Play strip poker
Listen to the birds (and the bees)
Play charades (nude)
Write a love song for your sweetie, and belt it out surrounded by candlelight.
Not so good with words? Just use an old standby and strum along.
Can't play? Fine. Call it a poem and recite. (But you might want to have some oysters on hand for an aphrodisiac.)