I am getting Married and the Coba on May 24, 11.
You are not expected to pay for anything or anyone. I attended a destination wedding last year as a bridesmaid and paid for my dress/shoes/etc...I am not paying for my bridal party, but will still give them nice bridal party gifts.
When my friend got married she invited people on an excursion as her way of saying thanks, not everyone will come. You have a large party, so no matter what you decide it will be pricey. You are right about the OOT bags, most people will have and bring their own items.
I would go with a gift for your bridesmaid, you already bought their dress, shoes, and jewelry.