The best thing about a day hike is you can carry anything you want. I have seen people whip out a full coarse sandwich and make everyone around them jealous. I have also seen a hamburger or chicken sandwich, saved from the Carl's Jr. and kept cool in a cooler from the night before.
Personally, I like to mix it up and carry a bunch of different foods. A few candy bars, like pay day or snickers, cheese and crackers, a fruit pie or two, raisins and peanut M&M's. I have also carried small apple sauce and pudding containers, Hot pockets, Dry carnation breakfast drinks with powdered milk, and beef jerky.
I only recommend to bring something you actually like to eat. Hiking Mt. Whitney is hard enough without gagging down some chewy dry power bar that tastes like flavored cardboard.