Day hikes are the ideal family activity: They are free, everyone gets to play outdoors, and the only athletic ability required is walking. But before you commit to several hours on the trail, make sure you've packed enough fuel to keep everyone going strong.

How much food do kids need?
Nutritional needs vary from person to person, but according to federal dietary guidelines, an active 5-year-old child needs about 1,600 calories per day, rising to 2,000 by age 10. Boys tend to burn more calories than girls, a discrepancy that spikes in the teen and young adult years. Females ages 14 to 30 need no more than 2,400 calories per day, while males between ages 16 and 25 need as many as 3,200. In other words, if you'll be hiking with a young guy, give him an extra cup of trail mix or a second sandwich.

Carbohydrates are the kindling in your body's campfire—great for getting you fired up—but you'll go up in smoke quickly without the slower-burning fuel of protein and healthy fats. As a rule of thumb, the Institute of Medicine recommends a daily intake of 0.8 grams of protein per kilogram (2.2 pounds) of body weight. For lunch, that translates to about 18 grams for the average adult man, 15 grams for adult women and teens, and 6 grams for kids ages 3 to 8. You'll get about 4 grams from an avocado or a tablespoon of peanut butter, and 21 grams from a cup of nutty trail mix.

Think sturdy
Most people think of sandwiches when they hear "lunch," but traditional sliced bread will end up squished and soggy after a few hours in your pack. For smaller kids, spread peanut butter and jelly on bagels, or make wraps with avocado, hummus, and matchstick-sliced veggies. Teens and adults might prefer Banh mi, a tasty Vietnamese-style sandwich. To make, cut a baguette in half and hollow it out a bit to make room for your favorite fillings. Traditionally, there's mayo and meat involved, but you'll want to avoid those unless you plan to carry an insulated lunch sack with ice. Try proteins like baked teriyaki tofu, or sliced avocado with quick-pickled radishes, carrots, and cucumbers. Season with salt, pepper, fresh cilantro, and hot sauce. When your masterpiece is complete, wrap it in foil and toss it in your pack.

Keep it simple
Children are especially prone to crashing from low blood sugar, so stop to snack frequently instead of saving everything for lunch. Fresh fruits like oranges, apples, and crisp Asian pears deliver a quick burst of natural sugar. Pack each fruit in its own Ziploc bag, sealing in extra air to cushion thinner-skinned fruits. These bags will also make it easy to carry out cores or peels without attracting bugs. Of course, don't forget the favorite of everyone from Girl Scouts to grizzled backpackers: gorp. Originally, this meant Good Old Raisins and Peanuts, but it's now used as shorthand for any type of trail mix. If your kids tend to be picky eaters, have a gorp-making party the night before your hike and let everyone create their own custom blend. Combine flavors and textures: cereal (Cheerios work well), dried fruits, pretzels, nuts, M&Ms, and maybe even some wasabi peas.

Treat yourself
If you suspect the kids might get tired or crabby during a longer day hike, pack something special that they can look forward to. Do they have a favorite candy that's normally off-limits? Remember, your bodies are working hard, so it's OK to indulge a bit. Some hikers swear by Snickers for a boost of energy that includes some protein. Healthier foods can be a treat too, of course! Sometimes it's all about imagination. Celery sticks stuffed with raisin-studded peanut butter sound a lot more intriguing when you call them "ants on a log," don't they? And broccoli florets taste better as "tiny trees." If a plain banana doesn't entice, try this idea: Spread some peanut butter on a whole wheat tortilla, roll it around a peeled banana, and call it a "banana dog." Cut it into slices, and it becomes "banana sushi."

Try this
My own favorite outdoor treat is something I learned from French backpackers: Tear open a chunk of baguette and shove some squares of dark chocolate inside. It's like a rustic pain au chocolat.