#1. What place is this? If it's on a dry dessert planet you don't want it to have lush green forests. If it's under water you don't want there to be dirigibles flying around. Make a landscape that fits what is going on in the game.
TIPS ON LANDSCAPING: I Think Low-Pollieness Is In Order
What is a "Skybox"? It is basically a box that has clouds or sky on it that you use to make the sky.
#2. Are there parts of your landscape that you will never get to in the game but still see? If so, photo and then delete them. They'll take up to much space and RAM. Instead make them part of the skybox.
TIPS ON LANDSCAPING: Death Defying Drops!
#3. Add some death defying drops and some ponds (but only if they fit the landscape). You may also add a "Physics Mesh" around a pond like this which is invisible but stops you from falling in.
Is your landscape High-Poly? If so you can make your high-poly objects "No Collision" in the physics and add a less high poly model which is invisible around it. This is called a physics mesh. It will make it so that the physics engine doesn't have to handle so much stuff.