This is something I feel is rarely tackled. Maybe because it is challenging, I don't know. So, let's see if we can change that.

Map something that is situated in a Marsh or a Bog. You can make a battle-map with several nice fighting areas. You can have a little larger encounter map that reveals several places that can be explored. You can map a swampy settlement. Or an overview map with many places to visit in the Marsh marked down. Imagine some swamp monsters lurking in old Ruins to lure curious adventurers.
This one has a lot of possibilities! So, be creative and bring surprises!