Wow that's a big family you have, extended family too I suppose. So what are your plans for picnic, is it like a one day tour or your are gonna stay there or something. Well let me tell you I know people who leave all these kinds of things to event planning. Event planners these days not only plan wedding or other parties they also organize picnics. I don't know if you want to leave your family picnic planning to event organizers, because there are people who are very nostalgic about these things. Otherwise you can look into that option, or else call upon everybody sit together if possible and plan the picnic, that itself would be fun.
