When I was doing something a bit like that, I found a lot of things in teachers resource books, after all they are set up for groups of children already. Try teacher's websites. I will try to dig up some if you like but I'm on my way out now.
Some unsolicited advise, read at own risk
I found that although my DS had a wonderful time - and still does - it wasn't really a learning experience for him. The other levels of the kids were just too different, even though there were some gifted children in the group. I'm not trying to make you feel down, I just had expectations that were too high when I started and found it quite disappointing. If I'd gone into it thinking "this might be fun" instead, things might have been less hard.
