I would find a lounge that you could buy access to and spend the time there. Given the vagaries of transport these days, your 6.5 hours may easily be truncated by 1-2 hours if not more.

You really don't have time to get into the city to do anything worthwhile such as go to a museum. And i am uncertain that it would be worth the cost to just go to a restaurant.

I am certain that you could get lounge access for a price.