My DD in particular prefers some things cold that I wouldn't eat cold. This was my meal list for DD to choose from for summer camp lunches- no heating available:
Edamame-cooked, salted, chilled
Hard boiled egg
Jelly sandwiches-kept 3 kinds of jelly as these were pretty frequent here
Fast Pasta salad-chilled pasta with broccoli lemon juice, olive oil and parm cheese
Mini bagels and veggie cream cheese
Veggie hot dog, no bun
She doesn't like hummus (she did 2 years ago, then suddenly no), but hummus and baby carrots would be easy/fast.
With any of these, I would add crackers, fruit/veggie cheese stick.
When it was a camp where she could take nuts/nut butter, we added mixed nuts or apple slices/pear slices and peanut or almond butter in a little container with a spreader.