In order of resistance:
To work your entire back at once (especially your lower back), just get on the end of a bench (or a couple of folding chairs) with your upper body hanging off (waist up), and have someone hold your legs down. Bring your forehead nearly to the floor (15-30 degree angle) and then bring yourself back to a flat position (back level with legs). If you want to add more resistance than body weight, clutch a weight to your chest while you're working. Don't add too much weight too fast, as you don't want to seriously injure your back. Start with your body weight only, you'll be surprised how much resistance that alone is. Holding a weight will also add bicep exercise to your workout in this routine.
Also, as Sparafusile suggested, you can do Supermans. Lay down on the floor on your stomach with arms out in front of you (like Superman). Lift your legs, arms, and chest off the floor slowly all at the same time, hold it for 10-15 seconds, then relax back to the floor.
And you can do Planks. Lay down on the floor resting on your forearms. Bring your body up to a straight line, resting only on your toes and forearms. Keep your back straight and your butt tucked in. Hold that position for 20+ seconds (however long you feel comfortable and can work up to). Then lower yourself back to the floor. This exercise will also work your legs, stomach, and arms.