One of the most important upgrades in Roller Coaster Kingdom is your parking lot. This should be your first priority before expanding your park, investing money in your rides, etc. If you don't have the maximum number of customers in your park then everything else is less important.
If you have the time, you should book the tours of customers that take less time to arrive because in the long run you will get more. However, if you don't have the time to spend hours a day playing, you should book ones that are convenient to your lifestyle, favoring the shorter periods.
