Reposting this list of places to build, now edited to take into account what people have built in the last few turns:
-By the restroom by Swinging Ship
-The space in front of Qin Dynasty, and also the space in between the tracks. Perfect for small flat rides, and can also fit a pretty big coaster inside of the space there, with a decent queuing area.
-The wood behind Motoplex, Skid Row Mice, and the village
-The castle
-The wood around The Dark Forest has some space
-The land in front of the Grand Tour
-The woods around Sioux Falls
-The empty land by Dangerous Flora
-The area around Mountain Splash has some spaces where you might fit some small flat rides and buildings in
-The space between Bouillabaisse and Shooting Star
-The space between The Stallion and King Of Durango Mountain (needs to be leveled if you build here)
-The riverbank
-The forest and grass around Project KB
-Behind Sherbywood Maze
-Behind Monster Jam
-Behind Ol' Timers
-The big hole (which you will probably have to fill in) in front of Turbo Tunnel
-Behind the Legendary Hammer Maze
-The mountain King of Durango Mountain is on has space for another coaster to intertwine between the coasters already there, with a rather small queuing area in front of it.
-In front of Shooting Star