Young person to bring in proof of- and prepare presentations on their hobbies. They then do the presentation in front of each other, and play games in between each presentation or block of presentations. Beavers can bring in their collections to show. Beavers group in lodges to display their hobby/collection. Lodges take it in turns to visit the other lodges tables. Each child can then talk a little bit about their hobby/collection.
Lay out tables to display hobbies/collection.
Send a note home to parents a couple of months before you plan on holding this event. This will give a child time to have sufficient time to build a collection or start a hobby if they don't have one already. I would then remind them a couple of weeks before to allow time to think of how to present it.