What would be the good exercise to remain healthy?

Exercise. Exercise is critical for good health. The minimal amount of exercise you should do is 150minutes/week (usually about 30 minutes 5x/week); try to mix it up between weight training and cardiovascular acitivity. I also think that adding yoga one time per week will help with the mind and body.
The one you like ! Moderate level (3-5 on a 0-10 scale) aerobic exercise lasting 20-30 min every day (non stop) is the key to better health. It doesn't matter if you walk, run, swim, use a 'machine' as long as it's moderate and non-stop.