I always liked Mexican food when the mercury soared. Not the kind of heavy stuff you typically get at chain restaurants, but good tamales, guacamole, rice and beans (not refried however) and fajitas.
You need sustenance to work, but too heavy a meal can hurt you in the heat. Good Mexican seems to be just the right combination of carbs and protein–just avoid the greasy, cheesy stuff. (And refried beans are a mixture of beans and Crisco–worst thing you could eat.)
On a related note, I heard a health expert once recommend limiting your consumption of sports drinks (like Gatorade and Powerade) to one for every three equivalents of water. Too much of the stuff is almost as bad as not enough.