My experience is that hostel guests do not expect something great (like sushi, thai food or something else), because most of the time they are out in the city exploring and finding out some good restaurants. In my opinion you should stick to breakfast. Croissants, oat meals, eggs, sandwiches should work. Or you can hire catering service and get 20% of orders for you.
Its much of a hassle to cook lunch/dinner meals, except if it is something traditional for your county/city.