Transitions
Avoid sudden, jarring transitions between theme areas. Note how the use of jagged lines, raised land, and fencing between the desert and grass (and on the opposite side, the use of water) minimizes the visual shock of crossing over. Notice also how the theme elements (tree types in this example) diminish in quantity and size as they get near the crossover, to further help blur the borderline.
Terraforming
Transform the Scenarios default landscape to incorporate land and water features that enhance the parks environment and theme. If a ride or path does not interact with the landscape feature, you can use "forced perspective" to make the feature seem bigger or taller. Note how the snow on the mountain top and the suggesting of a timberline causes the viewer to accept that the mountain is higher (and perhaps farther) than it really is. Landscape features can also infer what the world is like imediately outside of the playfield (for example, the back of this mountain is very easy to imagine as existing).
Theming
Although not required by the game, making theming associated with a ride congruent will make the park more visually attractive to you the gamer.
Decoration
Themeing elements are not limited to rides, but can also be used to dress up shops/stalls. Are your guests worthy enough to sit on this throne?
Color
Tasteful use of the Coloring Tool can allow you to turn the rides themselves into themeing elements.
Simplify Paths
The game A.I. performs best when paths are kept extremely simple, ideally with a single "main" walkway to which ride queues are attached.
Interweaving Paths
The game incentivizes players to interweave paths and ride elements (and elements from adjoining rides) together. Although not required by the game, attractive esthetics should be a consideration.
Crash
The sight you never want to see!
If your primary objective is to "win" Scenarios, then pre-built rides and cheap scenery is the way to go. However the game is also a visual feast for the eyes, so working towards a theme, or at least choosing non-jarring scenery, is encouraged. Here are some beutification and intriguing theme ideas:
Ride as a Diorama
"Dioramas" can be a lot of fun to create. A construction popular with styrene model builders, a diorama is a single scene used to show off a model (or grouping of small models) in a lifelike setting. Compare in your mind this picture of what could be your newest roller coaster all surrounded by themeing to the same roller coaster simply sitting on flat, undecorated terrain. Dioramas are particularly useful in Loopy Landscapes, where themeing around the ride can significantly increase the rides statistics. Tip: Because you cannot save the Scenery along with the Track, always make and save to a file several screenshots of the diorama (ideally from different angles) for future
Esthetics
Regular parks look more attractive when they too receive "diorama-like" attention to details. It can also put the park in contention for Awards (although Fences won't contribute). Placing complementing rides beside each other (such as the Hedge Maze and Miniature Golf in this picture) creates visual congruity.
Reduce Snow Monotony
Water features help to break the blue monotony of a Snow Scenario. You should also add patches of color, such as perhaps a shop selling Orange balloons rather than Blue. Note that the placement of stalls/shops directly next to each other in this picture is a BAD idea as far as the game A.I. is concerned.
Incorporate Humor
Entertainers in this scene are being used in a comical manner. Here a gladiatorial arena has been created for some Roman Entertainers and a Tiger. A pit was built with underground entrances, surrounded by a fenced viewing walkway, and the Entertainers placed inside. Selecting a patrol area for the entertainers keeps them inside the pit, where the appear to combat each other and walk in and out of the pit entrances.
Blend Queues into the scenery
While queue lines usually are located away from the main walkway, they don't have to be, and in some cases may look more attractive if they aren't. They can also be fitted with Lamps.