Have your friends over for a barbecue. Cooler full of drinks and a grill to cook the food, who needs electricity.
For those of us in the north light a fire in the fireplace, make smores and just enjoy the company of the ones you are with.
Have friends over and play board games by candlelight (or flashlight).
Camp out in the backyard.
If there is a planetarium near your home contact them. Many around the country are doing special star gazing events or go to for more information: http://stardate.org/nightsky/ or http://skymaps.com/downloads.html
Try and organize a neighborhood wide block party or barbecue and get everyone to shut off their lights. It would be a good way to get to know your neighbors right!
You could use the time to change all the light-bulbs if you have not done so already or make other energy efficient changes in your home.
If you have kids let them have a sleep over and do fun things with the kids by candle light to help them learn more about the environment. The EPA has a great website for kid activities http://www.epa.gov/kids/index.htm
Please post your good ideas of things to do during Earth Hour.