Remember what your mother used to tell you when you were a kid? That you needed to have a proper breakfast to set you up for the day? Well, actually she was right! You can't expect to do great exploits in the hills with no fuel on the fire. Whilst you may get away with a quick and fashionable expresso on the way to work in the city, in the outdoors it's a differrent matter.
Assuming that you probably don't want the hassle, weight and mess of a fry-up (lovely as it may be), cereals of some sort are likely to be the brekkie of choice. Oats are fantastic: they are rich in complex carbohydrates and release energy slowly over a long period (they are also good for protein and vitamin B1).
How you like your oats is up to you. Porridge really floats some people's boats and of course can be pepped up with dried fruit, nuts and so on. Instant porridge, or perhaps more correctly 'instant oat-based hot cereals' are available. If you are not a porridge fan, try a hot cereal which mixes oats and wheat, or is solely wheat-based: it may well be more to your taste.
Muesli is probably the commonest breakfast. There are lots of different types; I prefer those with no added sugar and definitely no dates (one of my bÍtes noir...)! A good tip is to make up single-portion packs of muesli in plastic bags (freezer bags or reclosable bags) and add dried milk powder to each bag: all you need to do in the morning is add cold water (a third of a pint, or a bit less than 200ml, is typical), stir, and Bob's your uncle! Do a bit of testing to get the right portion size: a little muesli goes a long way and there's no point carrying too much. About 80g works for me, you might have a bigger or smaller appetite.
An alternative is cereal bars: easier to eat, as well as tasty and nutritious if you choose the right one. See the discussion on Lunches and Snacks for more details.