Sitting for a long time makes for a sore back. I know, I had the WORST back pain from my long flights to and from Europe. So bad that I couldn't wait to get onto the airport floor and stretch.
That's right, my back pain bothered me more than the gross airport floors.
The reason sitting is so bad is that it's a fixed position, one which causes a lot of tightening to happen. The flexed hips/flexed knees mean that your hip flexors tighten up, pulling on your low back as they do, and so do you thigh muscles, especially those inner thighs. Your calves get tight from the bent knees and even if you aren't on a plane, immobile calf muscles make for pain in the lower limbs and wonky mechanics, which affect your hips and back.
SO. These are my go-to stretches to ease out the front of your body, calves and hips.
You'll recognise them all from yoga class: be sensible and don't do any if you know they don't work for you.
Cobra and two versions of Up Dog. There's a reason physiotherapists so often prescribe this pose to ease back pain - it stretches your front line and helps you find the natural curve of your lower back again.