Staying fit is a consistent process whereby you must engage in a certain level of activity everyday and have the discipline to control what you eat. While going to the gym three days will work many people fall out of habit quickly on their off days. A much better process is to integrate a certain level of fitness into your day-every day. This means changing how you workout to develop a "must do" every morning.
What works for me may not work for everyone. I work out 2 hours a day. I spend my early mornings, before everyone is up, at the gym. My basic workout includes 1 hour of weights and half hour cardio. When this is completed I feel as though I have done what is necessary for the rest of the day. I can add on more if I want.
At home I engage in yoga and kickboxing practices. Sometimes I might have a dance class or fencing. It all depends on scheduling, time, and energy. The yoga encourages stretching while kickboxing helps me to coordinate my body into a useful activity. Dancing provides a useful social outlet that burns a ton of calories.
If you do exercise everyday it is important to not over train. Listen to your body. If you don't overstimulate your muscles to the point that they are extremely sore you should be fine. At the point where you start feeling chronically tired do something less strenuous such as yoga or walking. Focus on other muscles that you haven't been focusing on as much in order to give a break to the over trained muscles.
People often complain of time. This is why I advocate for working out early in the morning to get the basics done and then focus on adding those components you can do at home. You don't need to do all your workout at once. You can break it up and complete ten minutes every hour. It is surprising how much you can add on throughout the course of your day.