In a good event plan, less is more. Avoid the temptation to make an event more complicated than it is. There is nothing more frightening than a four or five inch folder presented as a plan. If you cannot get the essential elements on one or two pages, you cannot understand and present the plan. A checklist is not a plan.
Check lists are important, but they are merely a collection of sometimes unrelated activities.
A good plan has a logical flow. A good test of a plan is to hand it to someone with no knowledge of the event or organization and ask them to read it. If they can read it and understand the event and the reasons for the event, the plan is probably sound. Avoid the temptation to try to make it perfect the first time. Planning is an iterative procedure.