We have a picnic social at my school each year, so to celebrate the picnic our class did a fun project. Each child was given a paper picnic basket to "pack" with paper fruit and lots of markers and fruit stickers. When they finished packing their picnic baskets, I hung their art on the wall outside of our class to get them really excited.
We also read, We're Going on a Picnic by Pat Hutchins.
From Amazon.com, "It was a perfect day for a picnic. Hen picked the berries, Goose picked the apples, and Duck picked the pears. Then they set out for the picnic. But young viewers will quickly discover that Hen, Goose, and Duck are not the only ones who like berries, apples, and pears! And as the picnic basket gets lighter and lighter, they will be laughing harder and harder."
Looking for a great picnic song, try Going on a Picnic by Raffi.