Download blender, and learn how to make more complex polygonal shapes. I haven't reached the stage of making anything like foliage or utilizing curves yet, but

Download blender, and learn how to make more complex polygonal shapes. I haven't reached the stage of making anything like foliage or utilizing curves yet, but just being able to make something like a door or a desk is liberating. Similarly, you can do a lot of texturing work with just GIMP and a phone camera.