Try Mill Creek Park. It should still be beautiful in September, although I don't know how much it costs. It would be worth a try! Here are pavilions you can rent with a number to call: http://www.millcreekmetroparks.org/RentaFacility/OutdoorPavilions/tabid/1595/Default.aspx
just found out about this place- seems reasonable.
Click on each tool to see a sample.