This is a very broad question and hard to answer.
What are your limitations on equipment? How long are they giving you? Do you shop yourself or do they provide the product? Can you order product or are you expected to just use what they have on hand? Where is this restaurant located? What is the style of the restaurant/what type of food do they do? How many courses are you doing? Apps? Entrees? Desserts?
How are you expecting to get an
executive chef job if you need 100% help coming up with dishes to do for a tasting? How would you write a menu if you got the job? I'm confused...
Some general advice:
Play to your strengths. Don't do dishes or techniques you aren't familiar with. You want tried and true stuff that showcases your ability to cook and season food, while minimizing the chances of a catastrophic fail. Concentrate on making a few good dishes, not many mediocre ones.
Think of what their current menu is and how you would improve it if you got the job, and use that opportunity to showcase your skills. For example, if they do a pasta dish using dried pasta, show them a similar pasta dish but make your own pasta.
Be ready for anything and have a game plan. Some places use these tryouts not only for cooking ability, but to see how clean/efficient/fast someone works. I've heard stories of people intentionally throwing curveballs last minute to see how the chef reacts--additional guests, last minute courses, missing ingredients, equipment not working or purposefully shut off, etc. How you interact with other employees is key--don't be snippy or rude to anyone, because they WILL mention it to the hiring managers.
I would recommend you give the forum more info so we can better help you. If you have a few ideas you're workshopping or whatever that might be a better starting point. Asking us to work a menu with you when we have ZERO context isn't going to work too well.