As a general rule, all workout programs should include some leg exercise. Far too often this type of movement is left out either because it is so intense that individuals simply do not want to do it or because they are so obsessed with getting their upper bodies looking good that they don't have any time left to do leg exercise.
One of the best leg exercise movements is the squat. If there is one strength training move you want to do, this is it. You will virtually work every muscle in the lower body with this as well as your whole core region. Make sure when you are doing it that your knees are tracking over top of your toes in order to prevent injury.
The second leg exercise to perform is a Romanian deadlift. This will help to develop your quad and hamstring muscles along with targeting your lower back. Both these and squats combined would really be all you need if you were looking to do a minimum amount of lower body work. This would add about 10 minutes to your workout, twice per week and is really not all that much to ask for when you think about how many benefits you will receive.
If you want additional movements, throw in some lunges as these target the glutes perfectly and will really help to firm up your back side and then also add in a leg extension and hamstring curl. These are both isolation movements that will really focus in on your quad and hamstring muscles. Finally, add in a calf raise of some sort, whether it is using a machine, a sanding calf raise or a seated calf raise.
There are multiple variations of the various movements as well. Squats for example can be performed with a wider stance, a turn-out stance, using a barbell resting on the front of the shoulders or using a barbell on the back of the shoulders (back squat). Varying the type you perform will help to keep your results coming and your body responding.
So don't leave this important part of your body out of your training plan ever again.