No, activities at brownies, etc, are much more prescribed.
This actually has an educational aim that the benefits get transferred to the classroom. The benefits revolve around the '7 cs' : co-operation, collaboration, co-ordination, concentration, confidence, communication, creativity and competence. In addition, the children are encouraged to take (reasonable) risks, and to use their initiative. We usually have an activity planned if the children want to do it, but we have found, as time has gone on, that they choose to do their own activities (which they base on the resources available - it's not everywhere you can sit in mud and mix potions!)
The children go to the same site every time - it is their outside classroom, and they are familiar with the resources there, in different seasons.
The added bonus is that the teacher sees the children in a totally different light. Some of the most reticent children become really rowdy and enthusiastic once the wellies go on!