Which Monday in September are we looking at? Labor Day, or after Labor Day?
If it's a Monday after Labor Day, in all likelihood (but not 100% guaranteed) the Halloween Time season will have begun. With that will come the Space Mountain Ghost Galaxy overlay and the Haunted Mansion Holiday overlay, as well as the Halloween Carnival & Round-Up (which is a festive area located in the Big Thunder Ranch section of Frontierland, featuring pumpkin carvers, Villains, games, entertainment, crafts, treats, etc.). There would also be a Dia de los Muertos display and the Ray Bradbury Halloween Tree in Frontierland (but not at the Carnival), and the "Pumpkin Festival" on Main Street (which involves 300 artificial pumpkins placed in various spots, such as atop awnings, on balconies, in windows, etc.).
At this time we don't know if the 60th anniversary entertainment is going to affect Halloween Time in any way, or interfere with decorations and the above-referenced highlights. Assuming that there is no significant interruption in Halloween Time fun, then be sure to check out those things that I mentioned!
I know that others will chime in with dining suggestions and 60th anniversary info.