Popcorn isn't just fun to eat, it's also healthy. Popcorn is a natural whole-grain food that's high in fiber. It's also naturally low in fat. If you pop and season the popcorn yourself, you can make sure that it's tasty and stays healthy. Corn popped in an air popper is good. Lower-fat microwave popcorn is also fine. Corn popped in a pot or popper gets a little tricky. To keep it from entering the "bad for you" category, cook it in a small amount of canola or other healthy oil.
You might be tempted to add butter. Don't. Butter would only make the popcorn sticky and messy at the beach, in addition to adding fat and calories. Instead, add a little Parmesan cheese, garlic powder, chili seasoning mix or other dry spices. Don't overdo the salt, because everyone's going to be thirsty at the beach anyway.
Pack individual portions of popcorn in plastic bags or containers with lids. You don't want a big container of popcorn open and vulnerable to sand or damp air.