Hi rebel,
Keep it simple and get the trains up and running. Astro-turf or poly felt fabric from Jo-Ann Fabric will provide a quick ground cover.
A metal trestle bridge with some back-painted 4mil vinyl for water flat will provide a simple visual effect. A tunnel is always a good element, as rockstars1989 mentioned.especially on an outside turn where the cars are viewed opened up. The train looks nicer on a inside turn. A station is always a good element With a little digging, you can find those used and rattle-can paint them for a quick touch-up if needed.
Some trees add a nice visual effect. A painted pine-cone doesn't look too bad. (Neil Young uses them)
Anything that suits your fancy will look fine. Something that relates to family members...kids toys(people, cars, etc.), any thing the spouse has (Dept 56) .
Get the whole family involved and have fun and get the trains up and running. It could even be on the floor.
Here's an oval from the free SCARM software.