Give them plenty of time to plan what they would do and then ask some pairs to explain their proposed methods to the whole group. At this stage, it is important to allow a substantial length of time for discussion. What do the children think the advantages and disadvantages of the different approaches are? Invite pairs to decide on a method that they think is the "best" and encourage explanations as to why. It will be interesting to note those who opt for someone else's method rather than pursuing their own.
It may be appropriate for you (or them) to find a suitable tree in the school grounds for them to carry out their plans practically. It is possible that difficulties might arise as they try out their chosen approach so watch out for the ways in which different pupils overcome any challenges.
It would be very beneficial to have a final plenary following on from the practical experimentation. How will the group judge how well their method worked? This would also be a chance for the class to discuss whether they would use a different method if given a similar challenge on a future occasion.