lower back stretches/exercises?

Looking for super simple and easy.  If it is too complex or time consuming, I won't do them.

http://www.youtube.com/watch?v=-67HQ5xetzk

Cat stretch is very easy, works your abdominals as well. Inhale the head up, exhale the head to the chest.

Here's one that's great for lower back:

http://www.youtube.com/watch?v=goL_5RASYog

You don't need to get your elbow all the way over, but do it as far as you can.

And one that I can't find a video of:

Laying on your back, bring your left knee to your chest and bring it across your body. Extend your left arm out, and look to the left over your shoulder. Keep both butt cheeks on the ground.

I would add 'Planks' to that list.

Get in the position like you are going to do a pushup but on your elbows, make sure your back and legs are straight, and hold it for 30 seconds (or 40 or 60).  That one works really well for me.  (3 'sets')

Also, and I know this sounds silly ... 'flex' your stomach muscles (basically feels like you are just sucking in your stomach) while you are doing some of these exercises, when pretty girls walk by, and even when you are just sitting at the computer.  I do that all the time, and it helps too.

Two exercises that I am doing following back surgery are:

isometric-style crunches, lying down with the legs bent, lift up into the crunch position and hold for 10 seconds. Work up to 20 reps. A strong core is vital for lower back support.

Kneel on all four and put the left arm and right leg straight out. Hold for 10 seconds, then switch to right arm and left leg. Work up to 20 reps. Strengthens the muscles that support the spine.

Planks, but sideways, strengthen the external obliques, taking a load off the spine when making turning motions.