Some mornings I just don’t have time to sit down for breakfast. I used to grab a granola bar, but they’re so high in sugar. What other quick, portable, healthy breakfasts are there?

Totally happens. In fact, instead of trying to sopisticate our morning routine to push into complicated stuff, working on our breakfast items would be a better idea.

For a quick grab, some nuts, almonds you can crush along the way on commute. You can also keep some fruits near at hand. At times like that I prefer to grab a banana or two. But you can grab any other according to your taste.
+Hope it helps.

If you have more time in the evening you could do some prep then. Make a chia pudding to leave in the fridge overnight, some easy portable egg muffins (though they are best when they are fresh, or if you can make time for frying some pancakes you can always make the batter then nightly before.