Identifying contour lines on OS map for certain area.
OS map of area being used.
Explain to cubs how the contour lines represent the steepness of the hill. (The amount of detail you use is up to you and the attention span of your cubs)
Using the map to identify the steepest parts of the area you are using, ask cubs to explore the area and to match up the map to the area which they have identified as the steepest according to the map.