A friend of mine does this with her children and I just love it. All week long children can earn 'money' to spend at the Mommy Mart. You could earn money by helping a friend or sibling with an activity, doing something without being asked, helping with dinner, reading books, cleaning up after yourself after a meal, or accomplishing a hard task. The skies the limit and can either be spelled out previously by an adult or can be awarded spontaneously. Keep increments small. Single 'dollars' or 'pennies' are a good way to go. As soon as you jump the amount to say $5 the $1 tasks aren't as appealing.
When it comes time to spend the earned money they get to go to the mommy mart. Items to by can range anywhere from an extra 15 of gaming time, or playing outside longer, staying up an extra half an hour, have a few expensive items too. Picking out a piece of candy at the store, getting to watch TV, picking what's for dinner. This also encourage children to learn to budget and save their money.