There are usually multiple strategies to successfully completing a scenario; these can be discussed on the scenario's discussion page or written down in an existing or additional section of this article.
For the three coasters, good advice would be to keep them small and not a complete circuit (a 'boomerang' design and a shuttle loop, both with the corkscrew coaster, work very well). You should complete the apprentice level in less than five minutes. For entreprenuer and tycoon, same idea, but you have to pay off your loan somewhere in there. To attract guests (hence money, build a boat hire between the two islands to give you a bit more room to work with, just watch out that you don't demolish too many trees at once. Also, make sure the boat hire is a complete circuit with two stations otherwise it will cost you too much in the long run. After that just build small coasters and platform rides on the larger island. Also, note that the objectives say the Maximum coaster speed is 51.45 mph, so be careful not to exceed that.