a FAVORITE...Sign In Door
One of my FAVORITE ideas for a Bar or Bat Mitzvah Sign In Board....Your child's bedroom door!
Just take the door off of the hinges, attach Sharpie pens and you have an instant Sign In Board. Many of my clients have "customized" their Bedroom Door Sign In Boards by painting them to match their decor and/or adding their mitzvah logo. After your celebration, just re-attach the door and you have a wonderful momento from your special event.
And this idea can be adapted for other types of events. I think it would be so cute for a baby shower...baby's first bedroom door would be full of wonderful wishes from friends and family.