This is going to sound like a very weird recommendation, please don't disregard it.
Selling on Ebay.
I'm a teacher (engineering) and it is something that I always encourage and help teach them to do.
Ebay is an amazing way to teach a person of any age (but especially kids) alot of valuable economic principles, while being exciting and multidisciplinary.
So many lessons are there when you post an auction. Supply and demand: your going to get more for your item if less are available and people want them. Sales and Marketing: do you make an appealing auction page? Do you tell the customer what they need to know about the product to feel comfortable about purchasing. Customer Satisfaction / Follow Through: Fast shipping, proper packaging, good communication after the sale? Cost of doing business, margins (credit card payments have fees, money order does not, etc).
There are so many things because you essentially run your own one-product small business for one week while that auction is running.
But the lessons go far deeper. The one that really had impact on me was the value of my material goods. I took care of my stuff as a child simply because my parents made me. But when I started selling items on ebay, that discipline netted me a large amount of money, with little effort. Every since I've started, I treat all of my stuff with the greatest of care, because it enables to me get top dollar for them when I resell.
And as soon as you start selling, you start buying. You realize that you don't have to buy from the top to be happy with the circle of goods. I routinely get items for far less than they would have otherwise cost me. Combine it with resale, and most of my hard goods are simply borrowed from the economy for a small fee and a marginal amount of effort. It greatly reduces my carbon footprint on the world, but still enables me to enjoy luxuries. Little of my stuff comes from the factory, and little of my stuff goes to the dump.
Combine it with the HTML and internet lessons involved, and there is alot to be said for Ebay as a lesson for students.
As an aside, although its a bit crunchy for my liking, this video always stirs up great debate in my classroom:
http://www.storyofstuff.com/