When that launch screen opens what’s your regular choice?

Scenario Editor

In Play, Sandbox. In Tools, Scenario Editor.

I am new to the game. Having just graduated from tutorial mode, I am playing through the base game career. I am sure if I stick with the game this will change. I like setting up the layout of the parks, but I haven't the patience to build coasters, so I like having access to downloads.

scenario every time

Scenario editor to make, sandbox to play.

just sandbox.

Well I use sandbox when I build stand-alone rides, but scenario editor when building parks so I have lots of space.

..... but I haven't the patience to build coasters .....

If you're new to the game, possibly you don't yet know the game comes with pre-built coasters, or that you can convert most of your beloved RCT2 coasters for use in RCT3.

I've had two coasters that wouldn't convert and I needed to use the Track Decoder to rebuild them from scratch in RCT3 but when they were done I was glad I made the effort.

There was one water slide ride that wouldn't auto convert, then later I had forgotten all about rebuilding, I got rid of all my RCT2 stuff and them remembered but the two rides I mentioned above were more important anyway.
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I'm almost always in sandbox playing an existing park but lately, while fine-tuning my next park, experimenting with blocks of scenery to save as structures and finding how everything works best with less real estate, I've been scenario editor a lot - but I consider this the exception rather than the rule.