Sliced cooked meat( I've just had some hot and spicy chicken slices from Asda and they were nice), cherry tomatoes, pickled onions, chunks of different cheeses, pate stuffed into celery or cucumber boats , same with cream cheese. Slices crustless quiche- lone quiche rush with sliced ham , fill with cooked onion, mushrooms, chopped bacon, pour over eggs beaten with cheese, tub cottage cheese and splash milk. Top with sliced toms. Cook till firm and golden. Can be microwaved till piping hot. Or make in bun tray so you've lots of mini ones- pop bit if cooked sausage or bacon etc in bottom of each bun then add egg mix. Cooked strips of bacon or sausage. Fingers of cooked salmon or those packets of salmon flakes or slices. Mackerel pate - simply mash with cream cheese and lemon juice and black pepper. Serve with crudités or little gem lettuce leaves as dippers.
If you have a food flask you could make soup and take that or even reheated portion of leftovers from night before- so veg curries, chilies etc