I LOVE this little sign that I made to put on our front door to let our dear neighbor friends know when the kids can and cannot play. It's been super helpful to have no interruptions while my kids finish their homework, when mom's napping and I can't deal with a houseful of kids, or any other time you need to regulate the coming of friends.
I went to the craft store and picked up a wooden door sign for $0.99. You could always make one on paper and laminate it so it will stand up to the weather too.
Pretty sure this has been one of the best crafts I have ever made!!