What you'll cook that day will be entirely customized to your interests and abilities. If you’re more of a beginner, you might start very basic, with knife skills, or push your luck and learn to flambé. (The chances are excellent that you will leave with all your eyelashes, among other body parts.) If your culinary skills are more in the intermediate zone, you might choose to focus on something specific: sauces, soup, desserts, etc. And if you’ve got a lot of kitchen experience, you'll really be able to stretch – one "Chef for a Day" opted to worked on the line during lunch service.
If, despite everyone's attempts to put you at ease, you find you’re still tense about messing things up as "Chef for a Day", you’ll find that cocktails from the bar find their way into your hand in the kitchen easily and often.
Be prepared for people to address you as “chef”. It is better if you manage not to giggle each time this happens. (I didn’t quite pull this off-- although perhaps cocktails had something to do with that.)
When you walk through the restaurant to go the restroom, or to the bar at the end of the day, expect diners to look upon you with a certain amount of reverence, brought on by that chef's uniform. Or maybe they're just wondering why you aren’t in the kitchen preparing their food.