Sandbox is a Game Mode found in RollerCoaster Tycoon World and RollerCoaster Tycoon 3. Many consider sandbox-like modes/scenarios that were present in RollerCoaster Tycoon and RollerCoaster Tycoon 2 to be sandbox modes. However, these modes did not give the player unlimited money, and they restricted access to rides and scenery.
In Sandbox mode, the world is nearly completely flat and empty, so that you can transform it to whatever you would like. You have unlimited money, allowing you to build an infinite number of attractions, hire an infinite number of staff, and terraform your whole world without having to worry about profit/income/expenses. Also, unlike Scenario mode, there are no missions to complete either in Sandbox mode. You have access to all of the content included in Campaign mode, as well as UGC, without having to unlock it. Sandbox is commonly played when a player wants to make a dream park of theirs, but does not want to have to worry about money and unlocking attractions.