sticker store
I set up a sticker store. Half the class sells stickers and the other half buys. Then they switch roles. I set up the prices and make signs before hand. I line up the large tables in the classroom and seat the sellers behind them with a pair of scissors (to cut the scissors apart) and a cup to collect the money. The buyers use our plastic coins to purchase the stickers. If they run out I give them more. They get to buy one sticker of each kind. They help each other count the amount.
The best part is they do all the work and you just help when needed.(Would be good for an observation). They could really make signs and come up with a price if you needed them too. Perhaps the day before they could do this.