guitarxe: I want to just get a land and build my park, without any mission objectives or whatever. Is there a way to play like that? Like a sandbox mode, or "free play" ? All I see are different pre-built parks with different objectives.
Of course you can, thats probably what most players do nowadays. You could grab the map editor and make your own scenario from scratch, including setting the goal to "have fun". Importing a save into the editor lets you change some things and clears the rides while retaining existing scenery to give you a head start at mapmaking if you like.
Or download someone else's "workbench", a scenario/map made for just that purpose, these sometimes have the goal of building ten coasters which comes down to the same thing if you never do it.
Unless you are set on a "don't let your rating drop" scenario you're free to ignore whatever goals there are; with that one exception, the game will not interfere at all if you ignore the goal and just do your thing.
A few links if you want to go the download route:
rct station don't remember much about those,
some nice landscapes to use in the editor, lunatim's
workbenchland is my default for an empty park when i'm too lazy to mess with the editor.