Fantastic moment for your son or daughter to bond/get to know the grandparent!
You can also have a night out whilst your grandparent looks after him/her.
Look at the positives- you WILL have arguments but these help smooth the ripples out to become a harmony. DONT bitch about your in law to your other half at ANY point. Talk.
I think this is great- days out, babysitting/bonding duty etc etc.
So what if your in law likes watching tv? At 65 not all of us want to be running up a hill. I know a girl at work who is 34 and doesn’t do any form of exercise. Her choice, her life. Who am I to cast judgement because I ride a bicycle (well push it up hills).