I feel your pain, however the good news is that 13 year old girls do not want to be activitied to death. They will find many things to do all on thier own. That is why you want to make sure there are no 13 - 15 year old boys around......
One of the best things I remenber is just discovering things on my own as I was growing up, and I have found with my kids, all four of them, the unsupervised, or so they thought, time was the most fun.
Remember how mature you thought you were at that age? Let them think that as well. Just keep an eye out.
Good luck! it should be fun.