Something better might be to say what NOT to bring:
-whips of any kinda
-a friend who is somewhere relatively close who knows what time you will be back and who will go save you if you aren't.
Do not bring with you anything that can be used against you or your horse. If your horse needs either of those tools to be ridden confront those issues in the ring until they can be ridden without them.
-a whistle, if anything happens you WANT to be heard, if it panicks your horse - well at least that got more attention (assuming the horse is uninjured)
-a cell phone (but signal may be an issue)
-light reflecting tack, so you stand out!
-bells if your horse can be used to it so hunters and others hear you from a great distance
-reflective vest for yourself.
And everything else mentioned :P