How do they ring up the customer's purchases? If it's a self-service restaurant, tallying up from what they see on a tray, could you have some sort of timed activity to test their accuracy and speed... like showing a tray full of food, then making it disappear and getting them to ring up the items from memory?
Or you could have a the keys of the till on a screen (using triggers on the keys and a numerical variable to total up the cost), then have the audio, or a video, alongside, with a customer ordering then, maybe changing their minds or adding items after they've totalled, etc, so they have to add or void items?
With customers appearing, you can animate in photos or drawings of each new customer, and maybe have this trigger a counter (again, using a number variable) just to add a bit of pressure.
Just a thought... but I've never done anything for this sort of industry, so that's just off the top of my head.