Whenever you can, go outdoors and for a walk in Nature. Find a bench in a quiet spot so that you can do some sitting meditation.
When you do sitting meditation the aim isn't to be like a Buddha, to solve your problems, or even to become enlightened. You sit to be happy. That is all. You sit merely to be there. You sit to be aware that the wonderful world is right there inside us, above us, below us, and all around us. If you can sit like that, happiness becomes a reality.
Sit for as long as you feel comfortable, whether it be fifteen, thirty, or forty-five minutes. Even if you only have time for a few minutes, enjoy it and profit from every single moment you sit there.
I find the best time of the day to do this sitting meditation is early morning. How many people get the chance to start their day so peacefully, sitting calmly and quietly at the start of their day? You will have many other opportunities throughout the day to sit mindfully, whether you're at home, school, at work or in the car. Just feel peaceful and happy as you sit.