Your best bet would be to call.
I believe the pavilion rental includes the guests
I believe they charge for daily rates. Different for overnight rates. But they might have group rates even. I'd try calling the park.
Each car entering for the day will need to pay a small fee for the day use. This applies to the pavilion or beach area.
Its not a party that will have alcohol.its a family party .my daughter is graduating high school.my daughter is not of legal age to even drink.it will be a day event.a picnic graduation party.
I believe you could rent one of the cabins for that, but the whole process for setting that up is beyond me. You'd have to contact them and inquire about it, since it is a state park they may have strict regulations when it comes to partying
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