I'm trying to come up with picnics in any of the books I've read...but nothing really comes to mind. I feel like...is there one in Eight Cousins? It's the only Louisa May Alcott book I really enjoyed when I was young and I seem to remember at least one meal outdoors with all those rambunctious boys.
It's not entirely a picnic but the first thing I thought of when I read this was the outdoor tea party under the cherry blossoms in Ouran Host Club. Here are a few lovely pictures from the anime of this part.