The simplest thing you can do to help your situation is to stop SITTING. Indeed, as your thread's title says, sitting can't be healthy: sitting leads to obesity, back pain, shortened lifespan, etc. Put your laptop on the floor or on the couch and get on your tummy. This very simply change will immediately make you feel better.

Not only do I use the laptop like that at home, my cats camp on top of me (because I am warmer than the floor, being a warm-blooded animal). So I get the benefits of touch, too, and am happy.

I am sure you did not expect a solution as simple as that.