Excuse the crappy mspaint pictures
Here's the scenario. You have a large outdoor environment. The area is littered with high-poly tree models, and there is a small stream running through the area. What combination of techniques is best for lowering the FPS as much as possible.
You are allowed to use anything that is useable in HL2, and are able to edit the picture to show hints and occulders.
Yellow: Tree Models
Blue: Stream
Black line: Displacement
For a bonus, place env_cubemaps in the optimal positions for reflections. Good luck.