Here’s a set of instructions for making some terrific looking craters. THe ingredients are primarily mdf, form board and spackle.
Modeling
Carving Sprue Plants
Here’s a neat idea for using plastic sprues to carve plants. It’s a good think to do with your old model sprues. But it strikes me that they would be somewhat delicate
Modeling Alien Trees
Dropship Horizon has a good idea for turning pine cones into weird looking alien trees.