Oatmeal with fruit like bananas (it can be thinned and blended)
Toast/Bun/or any bread you can warm or toast that has flavor by itself (you can still put toppings )
I usually don't eat or eat any warm (and unfortunately greasy) cheap snack I can get on the way, like meat pies, empanadas, muffins, and others.
Some stores sell very cheap on the go breakfasts. Some are healthy but most are regular. Breakfast burritos (eggs, beans, cheese, meat and cream) are the most filling.
They aren't typically healthy or small but they are quick to get. If you want a homemade healthy quick breakfast while still being warm, have something premade (like oatmeal you only have to heat) or something instant (like oatmeal from a microwavable packet). Or you can have something room temp while just adding a warm drink like tea.