Cardio and strength training are not the only things that will keep you fit. Stretching is important too.
Stretching improves flexibility and posture, gets your blood flowing, prevents injury and is good for the mind. The best time to stretch is when your body is already warmed up.
Spend five to 10 minutes stretching in a stretch, hold, relax pattern, without bouncing or pulling too hard. Stretching your low back by bending over at the waist and trying to touch the floor, with slightly bent knees and feet spread a little, is a good one to try.
The Mayo Clinic lists some stretching essentials here.