If you don't want to read this guide, then go experiment with some terrain, props, and nature!
This guide is for beginners who built a map, but have some empty space!

Terrain may be the best choice for this, depending on where your map takes place.

Terrain is easy to make, especially with the preset pieces.
Depending where your map takes place, you may want to follow up with putting more details on your mountain, hill, etc., maybe with some of the other examples on this list.

Grass may be another good option, depending both on where you map takes place, and how good your computer is.

If your map takes place in a desert of some sort, or some place that isn't... green.
you may want to make the grass a different color, with the color drawer.

You also may want to have different colors of grass in your map, making it look more realistic.

Now onto computers, depending on your computer, loading in many small props such as grass, may make your game run at a lower FPS, so if you want to fill up space, please be aware that this could crash your game. so either spread out your grass and use less, or choose a something else to fill up your space.

This is, by far, my favorite section.
This also depends on where your map takes place, and on your computer.

This is a good option for placing on terrain, and not as laggy as grass! so you can place as many trees as you want and not have your game lag too badly!
but still, if you place too many trees, your game will lag, I have never had my game crash from trees though.
You may not want to place as many trees in a dryer location as you would in say mountains, as trees don't usually grow well in dryer locations, but placing none would unrealistic, so you would want to have at least a few trees on your layout.
you can also color trees using the color drawer, which I talked about in the grass section.

These do not need as much discussion, so I will make this one section.
Neither of these lag much, so I won't talk about computers.
if you have a small or large space, these are good for both.
if you have a space that is road/path accessible and don't know what to put, try either a lake or pond of sorts, or a building!
if you have a space that isn't accessible, try a lake or pond of sorts!
if you have a large space, you could make a city!

Now, Don't put too many of these, if you're having trouble managing them, go look at other guides on track.

Sidings can be good ways to fill up spaces that are long and thin or next to stations!
Don't make them too long as it could get unrealistic.
Kinda a mini section that didn't fit in with Buildings and Lakes.

If you're still having trouble, ask me, someone else, or start experimenting!
Maps used:
https://steamproxy.net/sharedfiles/filedetails/?id=2126423870 Grass map
https://steamproxy.net/sharedfiles/filedetails/?id=2122781853 Lake, Siding, and Tree map
https://steamproxy.net/sharedfiles/filedetails/?id=2126427762 Building map


And that's all I have to say! If you liked it or it was helpful please like and comment on this! This is also my first guide so it would motivate me to make more.