While RCT2 has a 'hard limit' on buildings [128 rides as I understand - but I have not personally tested this], it does have a way to 'create a Sandbox Mode' - where you can build as much as you want, not limited by money - by making your own Scenario...
If you press on the "Game Tools" Toolbox at the Main Menu (initial startup menu, with the four large buttons to click on, at the bottom) and choose the "Scenario Editor" from the pull-down menu, you can create your own Scenario ("park level/map") and after setting up things like available Rides, Land and initial layout, you can choose for the Scenario to use "No Money" (a checkbox, later in the process), which is an option that effectively disables finances (and thus the need for money to build, etc), making everything 'free' when you go back and Play that Scenario.
There are also 'Trainers' you can run (which are small programs that run in the background of the game, that let you alter Money, let you bypass other normal limits of the game, etc.. [I haven't used these in a while, but have done so in the past]); but if you don't feel comfortable doing that (downloading a file from somewhere unknown and running it), the above seems to be the closest RCT2 gets to a true 'Sandbox Mode' that I saw, at the moment.
Have fun with it!
I had typed this up the other day and have recently been feeling like I have been 'fighting something' (an incoming cold/etc) so wanted to try finally finish it and quickly get it posted - so there are no Graphics/Screenshots at this time, sorry. I have taken a couple and may return here to add some in....Soon™