> It's been many, many years, but my memory is that while RCT didn't have a completely open sandbox like TT to boot, each scenario was effectively its own sandbox with restrictions that made them interesting

Also a very long time ago I played it, but I seem to remember that you could continue any scenario for as long time as you wanted after completing it, you didn't need to exit the scenario once completed. Maybe I misremember? If not, that'd basically mean every scenario is also a sandbox once the goals were completed.