Yoga tones and stregthens the lower back safely. Different poses are designed to work the muscles in that area as well as promote flexibility, reduce injury and enhance your mood.
Walking is one good way to tone your whole body. If your looking to tone your lower back weightlifting is one way to do it. Squats is another way to do it.
Get on all fours on the floor. Make sure your arms and legs are shoulder width apart, and your back is straight. Slowly arch your lower back, as a cat would do. Have someone put his hand on your lower back to make you conscience of the correct area you are to move. Repeat 10 - 20 times.
Most of the exercises that work the butt and thighs, also tone the lower back:Lunges, Squats, Dead Lifts, Stairs, etc. Combine with pilates and yoga for strength and flexibility.