Sculpey is a polymer clay that hardens in the oven at relatively low temperatures. It’s amazing stuff that can be used to create a wide variety of useful things for wargames. Here’s a tutorial on using sculpy to model slate.
Terragenesis has a neat tutorial for making a large vent stack — the kind that presumably would be used above a large underground complex. It’s a really neat model, in that it implies that there’s more in the area than what you can see on the flat tabletop.
I’m not one to spend a lot of time making trees for my wargames. I tend to buy the big bags of pre-made model railroad trees, preferring to spend my effort on painting miniatures or making buildings. But one thing that’s always been missing from the lines of plastic trees are birches. Youc an get them, but they’re typically very expensive compared to the plastic connifers and deciduous trees.
Terragenesis has the solution with a tutorial on making cheap (and what looks like quick) birch trees from fabric wrapped floral wire and floral tape.
The author didn’t put any leaves on them, but it wouldn’t be too much trouble with a can of spray adhesive and some appropriately colored foam flocking.