What do you always carry when you're going to be out all day?

I have Type 2 and most days my routine is pretty predictable, but once or twice a week work gets completely chaotic. I might leave home thinking I'll be back by lunch and suddenly I'm still outside at 6pm.

The biggest problem is food. I either go way too long without eating or get so hungry that I end up grabbing whatever is available. I've also had days where I walked much more than usual and started feeling shaky while I was nowhere near home.

I'm thinking of making a small bag that just stays ready with my meter, water and a few things to eat so I don't have to think about it every morning.

For those of you who spend long days outside, what do you actually keep in your bag? I'm looking for simple things that won't spoil quickly or take up half my backpack.