on the main page, the right most icon is a tool box, click the make scenario
Just set it so all the rides/scenery are there, make the map as big as you can, and put what ever you want in it. Then, when you get to the objective section, turn money off, and set it to a ridiculously easy objective, so you don't have to worry about it.
Another similar already in the game thing is the roller coaster editor under the tool box, there's no money, and it's just a big flat map where you build unthemed roller coasters and watch them go.