1) When stringing together lights for outdoors, it is a good idea to wrap the plug joints with electrical tape. This will help to protect you from electric shock.
2) Salt (sea) water and holiday lights don't mix. I found this out first-hand over Thanksgiving when we stayed in a beach house for a friend's wedding. Even though the lights were only up for a few days, the constant sea water misting of the lights that we hung on the balcony as wedding decor turned them into extremely dangerous electrocution devices. I'm not exactly sure of the science here, but I seem to remember from high school chemistry that salt water conducts electricity better than fresh water. After we were shocked...I looked up and down the beach and saw no other twinkle lights… and I think there might have been a good reason...
3) For similar reasons (danger of electrocution) you should never hang lights on a metal tree. Instead consider uplighting from a spot light.
5) Traveling by air? Did you know that there are special TSA approved baggage locks that you can use? If the TSA needs to get into your bag, they can, without breaking the lock — but no one else can.
And one last thing: nearly all the standard holiday gift plants (poinsettias, paper-whites, lilies, amaryllis, mistletoe, holly) are toxic to pets and often people as well. Take care when decorating to make sure that these don't get ingested and are properly handled.
Do you have any holiday safety tips to share?