[Rhizome]

I would think that something like setting up the ambush would be covered by Intelligence, Wisdom and Charisma. The Wisdom will come into play to help a character determine whether or not an ambush is a good idea to begin with, and then later if it is a good idea based on the plan figured out. I'd think that a character would use Intelligence to come up with the plan, either good or bad depending on the Intelligence score and rolls. Then Charisma would come into play regarding a character's ability to convince others that her or his plan is a good one. The more Charisma that character has, the more likely they are to convince the group.
Charisma is a tough one to RP, as it comes into play in RP situations in the interaction between two characters. My suggestion is to use you dice bags, even though there is nobody there to regulate it. Let's say you are tying to convince your party that the ambush is a good idea. So, explain your idea briefly and roll a persuade check. Then the people in your party can roll a will save. If the persuade is higher, then the persuasion worked. The others in your group might also want to put modifiers on their own rolls, which you should allow them to do for the sake of flexibility. For example, if you come up with the worlds worst ambush plan and try to convince people of it, it'll be far more difficult than if it is a good plan.
Anyway, just some thoughts. Hope they help. Talk to you soon.
Rhizome