Myself, I'm partial to Clif Bars, which have many of the nutritional advantages of other energy bars but in a more cookie-like format. The Crunchy Peanut Butter Clif Bar in particular is tasty ($1.35; www.rei.com). I also like Kellogg Nutri-Grain bars, available in the breakfast section of your grocery store. They have a fruit filling, are quite tasty, and have a decent nutritional make-up without too much sugar (more sugar than Clif Bars, though).
I also like bagels and peanut butter—really a good combination for high-energy activities. And bananas, lots of bananas. They're high in carbs and potassium. Lastly, I'm a firm believer in drinking something that has more nutritional oomph than water. Usually that means Fruit Punch Gatorade (the blue stuff disgusts me). But on a day trip from home, I also love to mix up a batch of pineapple-orange-banana juice (find it in the freezer section of your local store, near OJ). That's wonderful-tasting stuff, and really offers a lot to a hungry or thirsty body.
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