I know there are a lot of people that have not enough time to go to the gym periodically, even from my own experience, and I also know how important is to exercise a lot in order to keep one's body and mind in good health.
We can live without exercise, but we live better if push our limits daily - it's how are bodies are built: without physical effort we just wear down easier and die earlier. The Romans said “Mens sana in corpore sano” (healthy mind in healthy body) and they were so right 2,000 years ago. Things did not changes so much to invalidate that saying, so we must obey it as we can.
So, you are not going to the gym because you don't have the time? OK. So you must get the gym to you without getting all the equipment and filling your space with stuff that you might never use. Let's call it home gym, even it's not a real gym.
As you might already know or even practice them, there are a few exercises that can be done anywhere in the world unless something or someone is disturbing you: push ups, sit ups, jumping on the spot (with or without skipping-rope), squats and back extensions. Additional to these, you can use dumbbells and a boxing bag (if you can afford buying them and you have the space for them, somewhere in a room).
How to do push ups: lie face down with your palms or fists on the floor, push your body up with your back and legs in a straight line, lower it back down towards the floor, get it up again and so on. You can wide your arms apart to exercise your chest or close them together to strengthen your triceps.
How to do sit ups: lie on your back with your knees bend and your feet on the floor, cross your arms behind your head or across your chest and raise your body up to the knees and back to the floor. It's the same exercise you are doing in the gym, but without any additional equipment – maybe a bed or some piece of furniture to keep your legs fixed. One variant of this exercise is to stay on the floor holding your hands over your head on something to keep the body fixed, with the legs in the air, then move the legs up and down without touching the floor and being completely vertical.
Jumping on the spot (with or without skipping-rope) it's a great workout. If you'll do it often this you are going to get the heart pumping, improve your endurance, build your leg muscles and makes your body-arms-legs coordination a lot better better. While jumping without skipping-rope you can raise the legs up to the body.
Squats is a great exercise for strengthening thighs and shaping buttocks, it is done like this: stand up straight, bend your knees until your thighs are parallel to the floor. You can keep your arms behind your head or stretched outright parallel to the floor.
How to do back extensions: lie face down on the floor with your arms on your back, lift your head toward the ceiling as far as you can and lie back down on the floor again. Repeat it as many times you can, sometimes holding your up position for a few seconds.
Dumbbels are a good for strengthening your muscles, used alone or in combination with some of the previous exercises. Used regularly a few minutes every day can replace in some way going to the gym. Also, hitting the boxing bag will decrease the stress and will make your arms and deltoid muscles stronger. But you shouldn't do that without protection, you can easily damage the joints of your hands.
A left the best to the end, and I don't think that should be a surprise. The best exercise that reduce a lot your time inside a gym: running and swimming. They are also time consuming if you are doing them well, but they can only help you keeping your body healthy and strong.
