There are lots of spots for parking, one lot by the dining hall and another one near the gymnasium are two good options.
The ceremony will be indoors and the reception will be outdoors under a tent.
Semi-formal attire, just please keep in mind that the reception will be outside.
Link to September weather in Onekama: https://www.weather-us.com/en/michigan-usa/onekama-weather-september#climate_text_1