I have porridge but it isn't for everyone. It's an easily digestible food and so it can give you a quick spike. Avoid things like instant oats, quick cook oats etc., they are designed to cook quickly and they will be digested quickly. The best type of oats are pinhead or steel cut oats, the typical scotch oats, not to be confused with Scotts Oats, which is a marketing trick. Pinhead or steel cut oats take 45 mins minimum to cook but are full of fibre and, for many diabetics, won't trouble their BG levels. Some of the the rolled oats from health foods shops are quicker to cook bt you have to be careful. Best is to use a microwave and once boiling, don't cook for too long. A grainy chewy texture means that your digestive enzymes have some work still to do. A very smooth and creamy texture means that digestion will be very quick.
Müsli is very good as it is raw oats. You can add things like nuts and dried berries to it and add things like cinnamon or cocao powder for a difference in taste. You might get away with skimmed milk but to be safe, test. Otherwise, there are several milk substitutes around.