I happen to love Market Days. We do this every year. I've done Market days in the winter and also in the spring. After we do a week long unit on Consumers and Producers we break into groups to create a product or service to sell.
Here are some goods we've sold in the past:
chocolate dipped pretzels
chocolate dipped spoons
wrapping paper (butcher paper stamped with stamps)
Most items are around $1. This way the students don't have worry too much about change. We do it as a grade level but you could easily do it as a class. You can invite parents, teachers, other students or have it just within your class.
We usually plan about a week getting organized, producing our product, and marketing our store. I show my students videos about starting a business. Our library has a great selection. This year we even made a commercial using Animoto (free for educators) that was aired on our morning announcements. The students also hung up advertisements around campus to drum up business. This year my class raised over $300. We donated the money to a local charity. There are so many different twists you can do with Market! If you need any advice or ideas please shoot me an email at firstname.lastname@example.org.
I have a cute booklet on tpt that covers wants, needs, goods, services, consumers, producers and income.