My kids were having a hard time getting along at the beginning of the summer, so we tried the marble jars. Every time they did something nice for each other, they got to move a marble from the sad jar to the happy jar. 
Yesterday the last marble dropped into the happy jar, so they received their much-anticipated dart guns. 
Now that they have learned to be kind to each other, they are running around shooting each other with foam darts. Perfect.