I agree with Elizabeth, do a family day sail somewhere. Trying to move that many around for a day would be very difficult I would think. Another option would be go to Legalion for the day. You can eat there, have chairs, kayak, paddle board, and wind surf, the littles could go up the street and visit the butterfly farm (or you all could). But for a crowd that size I would contact Chez Pat (I think that's the name) and have it set up before you got there.