Keeping cool depends on the units - on older types, without A/C it's as TheEdge says - open the droplights on both sides, and turn the fan on. If you're lucky enough to have working A/C then turn it on and make sure the vents are open. On certain units, with the heating tripped out, the cab heater will blow cool air into the cab as well - a bit more effective on the fan in my experience.
As for healthy lunches, I'm not really the person to ask - especially when I'm spare as it's usually a trip to Greggs or McDonalds. I tend to just bring sandwiches when I'm out and about, or if I'm on early turns a couple of cereal bars and some fruit. Quite a few people seem to bring ordinary or pasta salads - though equally a number buy fish and chips or bring leftovers!