I never ate breakfast until I became diabetic, at which point my doc and dietician said I had to, but I also have to keep it to 40 carbs. I spent a lot of time researching and looking at labels in the store, and have come up with several things.
First off, eggs are free. I will eat a hard boiled egg now and again, but they're heavy.
Bagels are way out due to carbs, but bagel thins are only about 22 grams of carbs. Add some peanut butter or Neufchatel cheese and I'm well under 40 carbs. Substitute some of the English Muffin varieties, same thing.
One of my favorites is an 'everything' bagel thin, a slice of smoked provolone, a schmear of Neufchatel, and a tablespoon of my homemade tomato jam.
One Eggo waffle, a turkey sausage patty, and a scrambled egg on a weekend, with a glass of no sugar added 5 calorie juice. Sugar free syrup, of course.
A homemade Egg McMuffin (or biscuit) with bacon or sausage.
There is a company called Aunt Millie's bakery that makes a low calorie bread, and it's good. Most stores around me sell it, not sure if they're nationwide. Instead of 17 grams a carbs per slice, it's 14 grams of net carbs for two slices. So, a bacon or sausage sandwich once in a while, or just peanut butter on toast. Or, toast and an over easy egg.
And of course, oatmeal. Plain oatmeal (as previously noted) is boring. But plain oatmeal with cinnamon and a non sugar sweetener is pretty decent. There are a couple of the flavored instants I can eat, one is apple/raisin/walnut. I always add ground flaxseed to help with cholesterol, and it adds a nutty flavor.
One of my desk drawers has been relegated to pantry. I keep a jar of peanut butter, English muffins or bagel thins, and a couple of different oatmeal types. I usually eat breakfast at work. I also have some soups and other canned goods for lunch if needed, but that's a different thread!
Note that all of these choices are well below 40 grams of carbs, most aren't over 30, and some are below 20. This morning I had a slice of toast and peanut butter for a total of 15 grams of carbs. I could've doubled that and still been at 30, but I'm watching my figure!