yeah the paint effects tool has a large range of effects including fairly decent looking grass and trees. I'm not sure exactly what you mean but there are fully 3D trees you can just paint in and then convert to polys retaining all textures.

edit: (yeah again) just in case somone doesn't know, paint effects grass always looks better when it is converted into poly's. And if they become a hassle, just stick em into another layer, hide it, and work will become smoother