As per Zen philosophy doing nothing is meditation. Is it so?

Not being absorbed in any particular thing or idea, I think is meditation. Just observing movement without making more of them than just being phenomena.

Your wrong,
Zen philosophy describes the mind as a muddied river. When meditating, you clear the channels of the river and allow it to flow easier.
Its actually kind of a confusing analogy. But it works, when you think about absolutely nothing except your own breaths than it clears the mind and makes it more capable of action.

Doing nothing for the purpose of meditation can be meditation. Not vegging with a pizza.

The Zen philosophy of meditation is,
1. to calm your mind, and
2. to focus the mind on one thing.
If you sit down and close your eyes you will notice that your mind is jumping from one thought to another. Even though you want to concentrate on your breath you will find other thoughts also coming in between.
In learning to meditate you must first learn to calm your mind. You do this by following every thought that comes into you mind. You must be like a Manager of a Hotel who stands at the entrance and observes the customers coming in. Without getting attached to the thoughts try to observe them. You will find that a thought comes in and that it goes nowhere and another thought follows it – try following that too – with same results. This way you will find that your thoughts gradually subside. Do not try to force out your thoughts.
Of course do not expect results in one or two days. Practise it for at least 40 minutes every day for at least 2 months you find that you have ‘killed’ your train of thoughts. I have practised for 1 1/2 years and still vagrant thoughts come in.
Thereafter start concentrating on your breath. Notice the breath coming in and going out. You should not concentrate only on the tip of your nose. Notice the breath coming in and thereafter your lungs getting filled and then gradually exhaling the breath.
You should be able to get some guidance from the following site,
http://www.meditationexpert.com/
click on the free articles – other than Zen Meditstion it includes articles on all types of meditations including christian and muslim meditation.