the last time I was in PR looking for my wedding venue, we ate at this place called Cafe del Angel which is on Ashford Ave in San Juan and it was good. It's not a very big place but there was a good size party there. Maybe check them out because the food is very good and it was very reasonably priced. Or if it's too far/not big enough, you can always check out the restaurants at the hotels in Isla Verde. I'm getting married at the Intercontinental and we are doing something similar the day after by having everyone for breakfast at the Italian restaurant there. They are also reasonably priced and they are accommodating us, which is going to be around 100 people. Just some thoughts, hope it helps and good luck with the wedding!
A great location for a Brunch in Isla Verde is the Courtyard by Marriot. They may prepare a brunch for you in an event room that has a view to the pool or in a terrace over looking the ocean. Both views are breathtaking (contact Michelle Cotto 787-791-0404).
Or a bit further is the Hotel La Concha and they have a space for events that is located in the top of the hotel called "Mirador" and this room over looks the ocean and it is quite lovely (contact Fabiola Cruz 787-721-7500).