Sept is the slowest month with many hotels & places closed. BUT it is also the month of the national holidays so there is a lot going on as far as parades & "jump ups" (dances, celebrations). If you are flexible and open to ideas you will find a lot to do. Belize celebrates carnival in Sept. Many events will be on the mainland or in Belize City but you can certainly go over there for a day or 2 to check out the action. When the locals party, it gets wild and goes all night with no shortage of single females. But it is not what you would be expecting. If you are adventurous and ask the locals, you will find the best party you ever had!