I said this in another thread but I will repeat it here. If I had limited time to exercise I would spend 10 minutes a day doing plank exercises with its many variations. It is a full body workout along with mental toughness.
It works the core like no other... Inner core, outer core, upper, lower, obliques, your back... It does it all when it comes to the core. You can also lift one leg to work the quads of the planted leg and the hamstrings on the lifted leg and the butt. Your upper body gets a workout too... Arms, chest, back, shoulders, it does it all.
10 minutes is plenty even for people in great shape. Maintain perfect posture too, dont cheat to make it easy. Keep your head up and work that back... Keep the body straight to get them abs. If you can maintain perfect posture for 10 minutes then you are solid.
Work your mind too. Set yourseelf a goal a little out of your comfort zone and stay focused and dont give up. The target time is when you feel as if there is no way you can make the last 30 seconds... make it though.
Once you become solid you can use this exercise to really sculpt your body. Focus on a specific body part and how you want it to look and flex the muscle visualizing the fibers getting worked so that they can grow exactly how you picture them.
It is a great workout and it only takes 10 minutes. Try it. Search the web for variations, write the variations and times you are going to do, then use a timer and get started.
P.S. Dont forget to do a warm up before each workout. This not only prepares the body but it also gets the mind started on those days when you just dont feel like working out.
Make this a daily habit.