Many people substitute spinach for arugula but it is important to add something extra for flavor such as black pepper. A lot of supermarkets only sell organic arugula which can be quite expensive, but it makes for a great substitute in many cases. Both are leafy green vegetables which are delicious and can be substituted for one another.
You will find that there are many recipes where it is acceptable to do this, though you will probably want to add something extra for flavor. By doing so you will essentially enhance the overall flavor of the dish.