I'm playing the second alpha update for Planet Coaster, a theme park management simulation by Frontier Developments.
In its current version, the game allows for construction of a theme park on several pre-made maps. Terrain can be modified, and various rides, buildings, and structures can be placed. Roller coaster layouts can also be designed, and several theming elements are present. The actual management aspects of the park (beyond simply designing and placing things) are still in development. Planet Coaster is planned for full release later this year.