To me books and picnics just seem to go together. I have great memories of being with my mother and siblings laying on a blanket on a warm summer day with a great cooler of food besides us as our mother read to us.
Any good book will work for a book picnic, but it's nice to read a book that isn't dependent on pictures, that way the children can look up at the sky or follow an ant while the story washes over them. How about reading a classic fairytale or folktale. In the library, these are not shelved with the rest of the picture books, but rather with non-fiction (I know, I know, it's one of the quirks of the Dewey system), so you may have overlooked them. Next time you're in the library look at the JE 398.2 or J 398.2 section to find many lovely stories perfect for reading on a warm day, under an apple tree, while your children munch their sandwiches.
Here are some suggestions of folktale/fairytale collections:
"Mightier Than the Sword: World Folktales for Strong Boys", edited by Jane Yolen. Fourteen folktales from around the world that show there's more to might than muscle.
"The Adventures of Tom Thumb", by Marianna Mayer. Despite his small size, Tom Thumb is determined to do anything and everything that everyone else can do--and sometimes even more.
"Yummy: Eight Favorite Fairy Tales", retold by Lucy Cousins. Revamps eight classic stories, from "Little Red Riding Hood" to "The Musicians of Bremen," retaining all the emotion and humor from the original fairy tales.
"A Treasury for 5 Year Olds", edited by Daniel Howarth. This treasury brings together best-loved traditional fairy tales, favorite nursery rhymes,and exciting new stories
(Ah, I think I have time to take a book outside and read a bit right now.)