Well for a start you could bake your own traybake bars simply prepare a tray bake of whatever takes your fancy (fruits, nuts, oatmeal anything what can be traybaked) then when cooked you cut it in to bars and leave to cool, these will keep for a long time depending on how they were prepared then when you want to take them with you simply wrap in clingfilm to preserve their barlike shape.

Also you can easily if you carry a small satchel bag or rucksack carry sandwiches or salads prepared in a small pot just remember to carry a fork and some kitchen towel (pro tip, wrap the said fork in the kitchen towel to keep it clean in your bag and put enough towel in there to re-wrap the fork after eating so that your bag stays clean) You can also use this to carry foods such as pasta bakes, rice dishes (note, pasta bake when cold is delicious, some rice dishes are too but be careful some will be horrible and so you'll either need to think of access to a microwave or what works) ect.

Also learn to prepare haso-maki suishi rolls these can easily be put in a small sandwich bag or pot and carried around too providing you with an excellent source of food on the go. Alternatively learn to make things like pastry goods such as sausage rolls, pasties and pies alongside savoury goods such as scotch eggs these can easily be carried and eaten and depending on your ingredients list they'll be very healthy and nutritious.