Funny, because the Family Fun Jar actually does not involve a jar at all, but rather a little Ziplock container. We have a Family Fun Jar though and it is something we have been doing for about a year now. It is a reward jar. What we do is have the girls sit down and come up with a list of rewards they would like to win. Amazingly, they choose mostly things that are free! Lately they have chosen: bake cookies together, take a hike together, have a tea party, wrestle with daddy and a few others. We write their choices on pieces of paper and put them into the jar. When we "catch" the girls doing something good (above and beyond the norm.) we let them choose a slip of paper. The cool thing is, we usually do not tell them while they are doing it. My husband surprises them at some random time of the time. He will call them and ask one of them to stand at attention, then he makes a "formal" presentation of the jar and states why they get to pick a reward. It is really fun! Recently the Hippie was doing her nightly chore and when she was done, she asked for an extra chore AND completed it very well! She picked a "wrestle with Daddy" slip of paper. Last night the Genius got to choose because she cleared her sisters dinner dishes and was just extra helpful around the house. Her reward was a family hike. Last night my husband took them on a night "hike" through our neighborhood, complete with flashlights (ooh, aahhh). They had a blast! If you don't have a Family Fun Jar you might want to think of starting one. It is a really fun tradition!
Oh, and when we choose a reward we throw the slip of paper away so that we can add new ones when the jar is empty.