This Design Thinking practice is exactly what the title says - the team members have to imagine the box/packaging for a new product. They need to come up with slogans that correspond to the essence of the product/service, highlight the most important valuable features it aims at. It gets the team to think from a big picture perspective and from the shoes of the potential customers and to start designing a new product/service. In different formats you may either start with a blank box or provide carton to participants and ask them to design the box in the process.
Check out these great links which can help you dive a little deeper into running the Design The Box practice with your team, customers or stakeholders.