Scenery in RollerCoaster Tycoon is a tool that allows the player to add decor to their park.
There was not many scenery options in the first RollerCoaster Tycoon, but when RollerCoaster Tycoon 2 was released, many new scenery categories were added. An example of this is the addition of special walls and roofs. Also, the addition of the ability to change the height of an object helps the player to create structures.
Scenery can really affect a park. Adding a lot of scenery in your park makes guests think, "Great scenery!". If a lot of guests think this, the player will get the "Most Beautiful Park in the Country" award.
Putting scenery near some a ride can actually boost its excitement rating. This can be witnessed by clicking on a ride (a window should appear having the ride you clicked on it) and clicking the "stopwatch" button on that window to see the ride's stats. Keep that window open and open up the scenery window and start designing. As you design, the excitement rating will slightly increase.
Scenery can work for any ride (as long if you have space and money), but it's best to put scenery on custom-built rides and/or rides with low excitement rating.