I find it helps that I enjoy watching TV programmes like X Factor, Big Brother or I'm a Celebrity, always a good topic of conversation. Also, as mentioned above, just feed them!
When the GCs were younger I was always more comfortable doing something practical with them, which they loved, but I know what you mean about possible awkwardness when they are older. I am not a talkative person, and live alone, so it can be daunting to be solely responsible for conversation, not wanting it to sound like an inquisition. I remember how my grandmother used to sit silently in the corner when we saw her, which was embarrassing all round!
I sometimes provide transport for the kids, and on some occasions we will travel almost in silence, on others we chat non-stop. In fact a few weeks ago we got onto the subject of birth control, which I would never have anticipated! 
I would just suggest that you try not to worry about it, as the more you worry the worse it will seem.