I haven't any experience (yet) with grandchildren of that age, but if they are reasonably normal, perhaps you can involve them in washing up (make a game of it, singing a silly song together helps). Whilst washing up, conversations will inevitably crop up and you can ask them something about their interests and learn about them, and maybe they will listen to you if you tell some stories of adventures you had when you were their age. If washing up is not an option because you have a dishwasher, maybe getting them to help fold washing (it takes two to fold a sheet - right?) or do some gardening could start things off.
This depends, of course, on what you want from your time with them. A 12-year-old will have his own diversions and, as others have said, you should leave him space for them.
But if you want to talk to them, or just lark about, there will be a way!
Have a chat with your son/daughter (the parents) to see what they suggest. They may have ideas, but just don't have the time or the resources to undertake them with their children.
Once they realise that you are all theirs (unlike their parents, who are often distracted - we were the same) they may tear themselves away from the Xbox and do something with you.
I am hoping that, when mine are that age, I will be able to watch "Back to the Future" with them, and build obstacle courses for them on their bikes, go swimming, put up a tent in the garden and have tea in it or maybe go on shopping expeditions to buy books, footballs, shoes or jeans.....