Hey! I'm planning a wedding in Jekyll Island as well. A great place [if you don't mind an outdoor reception!] The South Dunes Picnic Area is lovely! There are two pavillions right next to each other-each seating 75 each! ALSO- there is the Beach Deck which is owned by the convention center that is right on the beach that would easily hold your number of guests. :D
Hope all goes well!
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