When you don't really have enough time to go to the gym (or like me - you just don't have one near to where you live and you don't have a car) - invest in your home gym.
Yoga mat (I got mine for 6 EUR), resistance bands (sale - 5 for a set!!!), stability ball and a jump rope. And you have all you need to start moving!
I got all those things (and the overall bill is still less than a membership at the gym) and I can squeeze in a portion of exercise anytime in between. I am independent from anything, I decide when and how long I train. And having so much to choose from at home, I do much more.