Learn about OS Maps. Find features on a map / understand the map key
Pile of identical OS Explorer maps, or similar
Show the Beavers an OS Map, and explain a bit about how they work. Explain the map uses symbols to represent things, rather than an actual picture/photo of them.
Ask the Beavers to guess what the symbol for a church, pub, campsite, hospital, etc. might be, and for them to draw their guess on the white board. Then show them the map key from the OS Map and see how close they were.
Then, put the Beavers into pairs and give them an OS Map (open the maps for them, and lay them out on the floor). Then ask them to try and find some of the symbols above on their own map.