I wanted to play the MegaPark scenario for RCT1 but was not finding an easy way to unlock it. One way is to load nearly complete saves for each scenario until you have unlocked the Mega Park.

Hope this helps others like me!

Or, you know, pass all original RCT1 scenarios (expansion ones aren't needed) :P

After checking out the Mega Park and finding out it wasn't what I was hoping for, I did some more searching and happened upon the world of "trainers" and "workbenches." After learning the hard way that DragonsIOA Trainer version 5 and 6 Beta 2 did not produce save files compatible w/GOG's RCT Deluxe, I found ZombieJoe's post explaining that Save Game Modifier 3.13 does. I also found a compatible, blank Loopy Landscapes workbench than can be further modified with the Save Game Modifier.