Porridge. Filling, has soluble fibre, decent source of protein, not terribly high calories. Get a large microwave safe container, put the porridge in (a serve is 40g), cover with 300ml of milk or water, microwave for 4 minutes in a 700w microwave, 3.5 minutes in an 800w microwave, or 2.5 minutes in a 900w microwave. (The first two times are instructions on my box of Quaker oats- not the cheapest porridge, but still cheap compared to other cereals- the final time is what I actually do every morning in my microwave). For simpler (but more expensive), you can get Oat so Simple or something like that. For more fibre, have a piece of fruit with it.
(It's a bit of a pain to clean up, but even if you don't have a dishwasher- I don't- bung it in the sink with water to soak it up and deal with it when you get back from work or where ever)