Homemade muesli (or even just good old porridge) is the best answer (never mind granola, most varieties involve cooking the oats to give them that crunch in a coating of vegetable oil). You control what goes in it and it does not have any of the added sugar, oil or salt that the majority of ready-made blends do.
Muesli will help keep you hunger-free for quite a while. This is due to the amounts of fibre, protein and good fats present in the various grains (oats, wheat, barley, rye etc), fruits (either fresh or dried) and nuts/seeds that you can put in it.
The following is a blend that I make in huge batches every so often:
8 large handfuls of rolled oats (not instant)
2 large handfuls of wheatbran (looks like sawdust)
1 handful of chopped dried apricots
1 handful of chopped dried dates
1 handful of crumbled walnuts
1 handful of smashed or chopped almonds, hazelnuts or Brazil nuts
Add the oats and bran to a big plastic plastic container with the apricots and dates. Add the walnuts and your other chosen nuts (I usually bash them up in a tea towel). This stores in a sealed plastic container pretty much indefinitely.
It's best if you make it up for eating the night before. I put about 1/2 a cup in a bowl then pour about the same quantity of milk in and let it sit overnight, covered in the fridge. It softens up the grains and leeches the flavour of the fruit right through the mixture.
In the morning you can add more milk if you think it needs it, or some yoghurt if you are so inclined. You can always put some sliced banana, grated apple or some berries over the top as well. You can also throw it in the microwave and heat it up if you want something to warm you up.
I really would not be worrying about low-calorie breakfasts since your body needs a decent sized meal at the start of the day in order to get it kickstarted, especially when you are considering increasing your activity level. The muesli is not what you would call a 'low-calorie' breakfast, but its incredibly healthy. No refined sugars, added fats or sodium. The wholegrains have a low GI and will keep you satisfied for longer. The good fats in nuts and seeds are extremely beneficial and will also lead to you feeling fuller for longer.