A picnic was a meal eaten outdoors in a park-like setting, usually on a blanket on the ground.
In 2269, a picnic was held on the Shore Leave Planet. Chocolate cake, oranges and bananas were served. Guests included Hikaru Sulu, Leonard McCoy, the White Rabbit, Alice, and a two-headed dragon. (TAS: "Once Upon a Planet")
In 2369 Lwaxana Troi organized a picnic for Odo and herself in a holosuite of Quark's at the location of Andevian II's fourth moon at dawn. When Odo refused she proposed that one of Quark's minions could bring the picnic basket to upper pylon three. (DS9: "The Forsaken")
In 2378, Seven of Nine was researching Human dating rituals to find an appropriate idea for her and Chakotay's third date. Neelix suggested she throw him a picnic, which she did in cargo bay 2. Chakotay told her she didn't need to go through so much trouble, but when she offered to find something less extravagant, he simply described it as "perfection." (VOY: "Endgame")