A practice I do most days, usually between 5-7pm (kidney time), which will be of help both for your back and flexibility (and kidneys at the same time!) is actually quite a passive exercise. It's designed to stretch out both your legs (alternating), back, shoulders and arms in one go. I have use of the bit of equipment in the pic (called a stretch bench of course!) but you can just as easy use a couple of chairs pushed together and the edge of a wall, or alternatively just lie on the floor with both legs up against a wall at the same time.
A video of someone doing this in practice with just a couple of chairs can be viewed from 19m15s here....
I spend half an hour on the bench most days (15 mins each leg) but I'd recommend perhaps starting at about 5mins each leg (or at the same time) and work your way up from there.
My hamstrings and back have straightened out a lot through doing this practice consistently - it's helped me to sit upright unsupported for longer periods of time too...... so can be of benefit if you regularly meditate also.
Anyway, worth a try..... if you give it a whirl, let me know how you get on!