What to eat before a race depends a lot on the race distance and your personal goals. Some people prefer protein rich foods or foods full of healthy fats to help satiate their "runger" (aka hunger caused by running), but I am sure you have heard of the term "carb loading". Carb loading is a strategy for filling up your body's glycogen stores before a race or run so that you have a quick and ready source of energy.
The way it works is this: your body's first choice of fuel to burn when you exercise is carbohydrates stored in the form of glycogen because it is the quickest burning macronutrient. Therefore, it is important to have full glycogen stores so that you do not "bonk" in a race - meaning your body has burned all of its "on hand" carbohydrates and your energy levels plummet.
Keeping that in mind, also consider how you have been fueling yourself for your training and practice runs. What does your typical diet each day look like? What foods do you stick to most often that you know will not cause your stomach to cramp or leave you feeling overly-heavy and uncomfortable? For the 5k distance in particular, you probably do not need to "carb load" so much the night before as you would for other distances because 3.1 miles just is not long enough to use all the carbs you would consume if you sat down to a heaping bowl of spaghetti the night before. It will probably suit you fine to just eat a bowl of oatmeal and a piece of fruit.
The morning of the race, do NOT try anything different than your used to! Experiment with eating different foods before you training runs to know how your body reacts. Need an idea? Try half a bagel or a banana topped with a tablespoon of peanut butter, or a boiled egg and a piece of toast. It is good to aim for all three sources (fats,carbs, and protein) of macronutrients in your pre-race meal, with an emphasis on the carbs.