These stretches work mostly the lower back, but also make some demands on your abdominals, and your external obliques (sides).
Lying down with your back on the floor, straighten one leg, while bending the knee of the other leg, and try to bring the thigh of your bent leg as close as possible to your chest. Hold it there for 10-15 seconds. Then cross your bent leg over your straight leg and try to touch your knee to the floor (while trying to keep both shoulders on the ground). Repeat this same procedure with the other leg. Then, bend both knees and bring both thighs up against your chest (keeping your back on the floor). Hold that for 10-15 seconds. Then, put both feet on the ground but keep the knees bent. While trying to keep both shoulders on the ground, roll your legs over to one side and try to get your knees to touch the floor beside you. Hold for about 10-15 seconds and then do the same thing on the other side. Now repeat the same stretch, but this time begin with your feet off the floor so that your leg is bent at the knee at about a 90 degree angle.
As for isometric stretches for the back, I don't recommend them.