First is a patch of mushrooms in a room corner. Instead of going for crazy, bright colors, I decided to aim for a more realistic hue. I also added some mold around the bottom to hide the bases and spread it to the walls to increase the grossness factor. The mushrooms are from a GW night goblin sprue I picked up a while ago.
The other room corner I finished contains a rat's nest, partially inspired by Diablo II (I fondly remember kicking open a lot of them looking for loot). It also makes sense, since a dungeon would have its fair share of regular vermin in addition to the monstrous type. It is made from a couple of broken up pieces of wooded coffee stirrer with dead-colored static grass all over. I only wish I had an appropriate sized rat to give it more context, but I'm still looking.
I decided to use the remaining pieces of broken coffee stirrer to add some debris to the floor. I figure there probably aren't a lot of hobgoblins with brooms roaming about to clean up. I made sure to keep the pieces out of the middle of the squares so miniatures could stand in them, but they aren't so high that they couldn't be stood on. I definitely plan to add more of this sort of thing to give some extra life.
And mixed up with some of my previous pieces: