I pick through the dollar store or the birthday treat-bag area of Target/Walmart to load up on little, inexpensive toys for this activity. I set prices for the different items that are manageable, depending on the ability levels of your students. I prepare zippy bags with a variety of coins (pennies, dimes, nickels, and quarters) so that the students are forced to think through all choices when they visit the store. They get an "allowance" of anywhere from 50 cents to one-dollar to come visit the store. It's a great way for them to see what their money gets them, how change is made, and how to count out what they need to give the cashier (you, or a parent volunteer).
*example price list/toy list
*follow-up activity where students draw the coins they used and show what they purchased