I have a few good go-to items that make this easy, I'm gluten-free and vegan, which seems like it would be impossible while out, but it's really very easy. If in doubt, there's always fruit if I need something sweet on the go, nuts if I need protein. Smoothie bars make very good options. And if I need a meal while I'm out, Asian places are usually much easier than others to find something gluten-free.