One potential problem with simple foods (that I've made for myself in the past) is not having enough salt in my diet when sweating a lot. Remember your electrolytes!
Cold-water-soaked oats with some fruit can be a great way to get morning carbs, I think. (Shelled) sunflower seeds and nuts are tasty and reliable calories that can last a long time. Canned (or otherwise preserved) fish can be another, if you like that. I like tuna and whole-grain mustard and green onions in a pita, but I know 'tis not for everyone. They make canned chicken and turkey, too.
And simple foods don't have to be boring: Any number of herbs and spices can work well with uncooked meals. Search the web, experiment, make mistakes. Have fun!