If you were to ask my mother-in-law, she'd tell you to put a baby in it.
But I digress...
You could turn the room into a casual family room. Or you could turn it into a hobby room. My house has three bedrooms - a master bedroom, an office, and a guest room. (We have no children.) We usually refer to the guest room as "the yarn room" as I am a knitter and that is where I store my yarn stash.
Or, as you mentioned, you can just leave the doors open and have essentially one big living room.