Cans of diet coke are a really poor idea above around FL50. The lower pressure causes them to go everywhere when you open them, crisps are also not the best idea. They go everywhere, and make loads of crumbs.
Anything resealable is much better, especially 'sports caps' on water bottles. Fairy cakes are always good, but dont stand up to aerobatics very well, as they squash to easily, and become all deformed as they bounce around your lunch box.