Using old Christmas balls for Halloween is a unique idea. For additional information about decorating, please see the following website: holidays.thefuntimesguide.com/.../halloween_decorating_ideas
The websiting for Southern Living has a video for "100 Ways to decorate for Fall, Halloween and Thanksgiving" that has some great ideas. http://www.southernliving.com/home-garden/holidays-occasions/fall-decorating-ideas-thanksgiving-halloween-00400000055475/ Other home websites like HGTV and Better Homes and Gardens have great ideas for fall and Halloween decor this time of year.
The reason people decorate their houses for Halloween is because It is showing other people that Halloween is fun to celebrate and to show that when you go trick-or-treating, you can stop at that house for sweets!
You can find decorations for your door online at this site: http://holidays.thefuntimesguide.com/2009/09/halloween_decorating_ideas.php. It has lots of ideas, as well as some videos of ideas, not just for decorating the door, but for decorating other areas of the house.
If you haven't been to Bizrate online, then I suggest you head over there. They have witch lights and many other styles to decorate with for halloween. Here is a link to their selection to help get you started; http://www.bizrate.com/halloween-decor/witch-lights/.
How to decorate for your Halloween party really depends on your budget. You can go all out and get talking statues, light up skeletons and the like. A fog machine and spooky music playing will add to the ambience. Keep the lights dim and use black lights and colored bulbs. You can purchase a cauldron from any halloween store for punch or other snacks.
You can get some great ideas for scary Halloween decorations from a book called, "Extreme Halloween: The Ultimate Guide to Making Halloween Scary Again," by Tom Nardone. Inside the book are ideas for creatures to make and other ideas such as pumpkin carving.
Christmas centerpieces such as candles can be re-used at Halloween if they are neutral colors such as white. Also, crystal candle-holders can be used if you add other Halloween-type decorations with it.
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Walmart and Target sell foggers for a relatively cheap price. Most halloween stores will also sell foggers, as well as other halloween items besides costumes. Halloween USA is a great store to buy a fogger from.
You can find ideas on how to decorate a headboard online at the Better Homes and Garden website and HGTV's website. Other good websites to consult are Houzz and Remodelaholic.
Halloween party games are always a fun addition to Halloween parties. I would suggest checking out www.halloweengames101.com. They have ideas such as apple bobbing, card games, among many other fun ideas.
You can decorate by splitting the house into two parts. one half of the house can be dedicated to the boy and the other can be for the girl. This way each child gets a party.