Nuts, leftover cooked chicken or meats (I like roasted chicken or grilled flank steak best), cut up veggies like celery, carrots, cucumber, peppers, radishes, jicima, cherry tomatoes, hard boiled egg (although this might not make your seat neighbor happy), homemade jerky ....
I have never had a problem with foods going bad in my purse for a full day. I try and eat the ones that would 'probably' go bad sooner....so meats first, veggies second, nuts last sort of deal.
I have actually gotten a fruit and cheese plate on a couple planes lately (tossing the crackers). Not great, but sometimes those are offered.
You could also incorporate a fast in there.
Good luck, stay hydrated!