Hi Megan! Like you, I enjoy getting the most bang for my buck, so trust me when I tell you that you shouldn't feel cheated because the Magic Kingdom closes at 7 p.m. during your visit. Walt Disney World park hours vary throughout the year due to many factors. One major factor is park attendance levels. During the summer and on holidays, the parks are open longer because they are much more crowded. Your trip in mid-September falls at a time when attendance in the parks is at its lowest levels of the year. (The website TouringPlans.com has a Crowd Calendar that predicts park attendance: summer crowd levels rank a 9 or 10; predicted crowds during your visit are 1 and 2.) Because the parks are so uncrowded in September, you'll face little to no waiting for attractions and will be able to experience just as much or more of the park in a shorter time than summer visitors!
To make the most of your visit to the Magic Kingdom, be sure to arrive at the park at least 30 minutes or more before the 9 a.m. opening time. That way, you'll have time to make it through the bag inspection and the turnstiles and you'll be one of the first people into the park. You'll be amazed at how many rides you can enjoy with no waiting in those first few hours! When the park closes, you'll have time for a nice dinner and a swim in the resort pool.
Also, don't forget that Disney has three other parks in addition to the Magic Kingdom. If you have a park hopper pass, you'll have the ability to visit another park after the Magic Kingdom closes. If you don't have the park hopper, consider visiting the Magic Kingdom on Sept. 9th or 12th when the park's open later and there are Extra Magic Hours for Disney resort guests. On Sept. 10th, go to Epcot which will be open until midnight for resort guests.
Megan, please don't worry -- you're going to have a magical trip!