A couple of lower back exercises I know:

Dead lifts. Bend down at the waist picking up your weights and keeping your legs straight (but be careful!) slowly lift the weights by straightening up to a completely upright position.

Lay on the floor face down with your legs straight out and your feet together. Place your hands under your chin. Slowly raise your upper body, keeping your legs on the floor. Hold for a few seconds and lower back down.

Paleoanth