A large pomegranate made from painted bubble wrap, where each bubble stands for a seed and a child pops one for every good deed. It turns the idea of a year full of mitzvos into something a child can see filling up.
Three to about eight. Younger children need an adult to cut and paint while they do the popping; older ones can make the whole thing. The popping is what holds the attention, and it holds it far longer than the ten minutes of setup suggests.
About twenty minutes of work plus drying time for the paint. It is worth making it a day or two before Rosh Hashanah so the paint is fully dry and the pomegranate is on the wall before the holiday starts.
Hang it where children pass it — a kitchen wall or the back of a door. It runs through the Aseres Yemei Teshuvah rather than one day, and finishing every bubble by Yom Kippur is the goal that makes it work.