What sandwhich filling is best for DofE?

Next weekend I'm doing my camping course or whatever (D of E) and we need sandwiches for our lunch. I'm going to bring pittas, but I'm not sure what type of filling to put in it that would be able to stand warm temperatures, or a while of the shelf and in a bag. I know that cheese can 'sweat' and meat goes off, so what other fillings can I use that are tasty, don't smell (ie. tuna) and will give me enough energy to walk for 9 km?