3D "Fly Through" Level

I’m trying to create a 3D environment that the player flies through. Here’s a rough sketch of what I’d like, and a screenshot of the game so far.

The overlay image is a scene of 2 aliens inside a spaceship, this is stationary. The blacked out windows are where the player can see the 3D environment.

I know I’ll probably have to use a 3D box for the floor, and the billboard extension for the trees. My questions are:

  1. Will the 3D box work as a floor that the player camera can move through
  2. How can I place the camera on an angle so it level with the ground and let the player move it up and down as it moves forward

Any help on tips or directions I should take to achieve this are appreciated! I will post back with updates.