A great fitness tool I recommend to everyone is the stability ball, also called a physio ball or Swiss ball. Personally I think it should be called an "instability ball" because it rolls around and changes shape while you do the work of providing stability. The stability ball is just a large, inflated plastic ball, but it can be used for back care, stretching, exercise, and a place to sit. Besides versatility, they are also inexpensive. There are many ways to use them, below are my favorites.
Sitting and Back Care
Sitting is a necessary evil for most people. One problem is the poor posture we tend to assume while sitting, the other is the lack of movement. While sitting properly and ergonomic chairs help, no position held for a long period is healthful. Sitting on a stability ball may help you be in good posture, but the main benefit is that it will keep you from sitting completely still. Muscles and joints get subtly(or not so subtly) worked. The low back particularly will be helped.
An excellent stretch to counteract rounded shoulders and upper spine is to lay supine on top of a stability ball. Squat on the floor with the ball behind you and slowly lean back onto it. Raise your arms up and behind you, and roll backwards until it is under your upper back. The feet stay on the ground.
Your weight is supported, so your muscles can release and you can stay here without effort.
This stretch is not for anyone with an injury or compromised spine, or without the strength to control your position. A larger ball, 75mm and up, will make the stretch more manageable. Putting the ball against a wall so it can't roll away is a good way to start. This modification should also be done by anyone with hyper lordosis of the lumbar area, to keep from increasing the flexion there.
A stability ball can be used in place of a bench or other rigid support to get much more of the body involved in your exercises. Even for familiar exercises, more muscles will be recruited, and most importantly, they will learn to work together instead of isolation. Additionally, because the ball is compressible, the exercises are more dynamic with the force applied creating less stress.
You may find you can lift a lot less than with the same exercise done on a rigid support. Good! That means you're using more muscles that don't get worked with common gym exercises, developing balance, and not propping yourself up with furniture.
One exercise I suggest is a reverse dumbell fly. Lay with your chest on the stability ball and a small dumbell or medicine ball in each hand. From a slight spinal flexion(forward bend) go to a slight spinal extension(backward bend.) With the ball lower on the torso you would be working the glutes, hamstrings, and low paraspinals, but I prefer to make this an upper spine exercise. At the same time you're straightening your upper spine, bring the shoulders back and arms up. Make sure the shoulders are retracting, meaning they are pulled together toward the spine, and the top of the spine is pulling back. Done this way you will be working the muscles needed to counteract the typical rounding forward of the upper body from sitting at a computer.
So, introduce some instability on purpose, and prepare your body for when it happens as a surprise.