Got invited to a wedding in Cancun. The invitation said "Frat Boy Casual". What does that mean? What should I wear?

My fiance and I are eloping in Costa Rica and it will be just the two of us. We want to have the ceremony in the afternoon so we can take pictures during sunset on the beach but that leaves the entire day to do something. It won't take us very long to get ready for the ceremony and since we are in another country by ourselves we wanted to make the entire day special and spend it together until we are getting ready. Ideas for what we could do? It will be in January which is dry season in CR and it will be good weather for being outside.

We recently got engaged on our anniversary and plan to marry next year on our next anniversary. We already planned on a small wedding with no bridal party because neither of us have many close friends. We were just planning on our immediate family, a few aunts/uncles and cousins, and some friends. However, we want to have our small ceremony 2 hours away from our hometown. His mother said no one will attend because they don t want to drive the 2 hours. Our venue is in a very popular tourist area with lots of hotels available and many attractions and restaurants. Not a terrible place for a wedding. Because of negative feedback I m now thinking him and I should just drive up ourselves, have no guests at all, and the chapel can provide witnesses to sign our license.