This lesson is designed to take from 1-2 days (depending on length of class periods) My students had studied discounts, tips, tax for about a week in preperation for this activity. I had students work in groups of 3-4 to allow for collaboration and discussion amongst each other. This was helpful when many students had similar questions.
I set up four "stores" around the room. I printed out signs to designate each one and also printed out the weekly ad for each store (taken off of the internet). I then made up four discount signs and hung them at each store. (i.e. I made one store 1/4 off any price, another 33% off, another .5 off) Students looked through the ad and selected two items to purchase. They filled in those amounts in the first column and calculated how much money to take off that price. I set the tax at a friendly 5% and students calculated how much money each item cost in all.
Follow ups to this lesson can include: putting restrictions on the amount of money they can spend, giving them a family and seeing how much groceries, etc can add up to, or you can differentiate and create homogenous groupings with a more restricted selection.