A cash register and some pretend food (or keep old food cartons etc). We often play this game, i pretend im in the shop (boob hanging out feeding my DD#3 mind you ) and my DD#2 pretend she is the shopkeeper and asks me 'what you like mummy?'. Its a fun game, and i would think at 18 months they would have a general understanding.

All my girls still love putting their babies (dolls) to bed......along with pretend feeding equipment (spoons, bottles etc). They can play for hours........