You really can't ask that. If they are completely utterly shy then they would want there space and if there hyper take them out somewhere like a park. But it depends in their personality. If your lucky they may be one of those demanding kids who make your job easy by just telling you what they want to do. Hope this helps ;)
I cannot say much for the boy, but I remember from when I was twelve I was babysitted by my brother. He let me hang out with him and his friends and kept me entertained. That is what you need to do; make them feel like they fit in with your social circle (but don't talk to your friend to much 12 year olds tend to get jealous 0.0), spend a lot of time playing with them and hanging out with them, and keep then happy, but do not spoil them. Also, I am almost positive the boy will get bored if you spend activities only with the girl, so do activities all together!