Hmm, do you mean how they are still buzzing from their vacation? I would probably get them to briefly talk about what they did and interweave it into the lesson plan.
For instance, "Do you know why we had yesterday off?" - and that could begin the history lesson. Children and adults aren't very different, when you think about it. Adults want to talk about what they did over vacation, and so do the kids. They aren't going to want to just *hit the books* right away. I think a savvy instructor would use a clever segue so that the process looks more or less seamless to a casual observer.
I think it's easier to get the kids to gear into the lesson plan when it seems relevant to what they recently experienced.