Used to live across the street from one of the sites of NYC's version of the event you're going to; always fun

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I find the easiest picnic meal to make when time is short is an antipasti platter pulled together with pantry items and a stop at a deli counter.
A jar of marinated artichoke hearts, a jar of whole roasted red peppers cut into strips (truly fast roasting a few peppers at home plus you can add to the prettiness of the platter by using different colors but if you don't have the cool off time, do the grocery shelf route); Giardiniera salad. Stop at the deli counter and get some Italian dried salami, mortadella, prosciutto or a good quality boiled ham, whatever soft and hard Italian cheeses looks good (imported Provolone, Asiago, Peccorini, or Parmesan; ciliegine or bocconcini are all good possibilities) and whatever olives that appeal most to you. Drain the jarred goods and place on a platter, interspersed with the meats, cheeses and olives. Over-wrap top to bottom and over again so the food doesn't move and place in your storage bag with a few frozen ice packs.
For dessert what with many being at the height of the season a bowl or platter of fresh fruit could suit your parties needs. Watermelon cut in to triangles (easier to eat with the rinds on), whatever berriestaste good at market, same for stone fruit. To drink add a bottle or 2 of cold Prosecco or Chianti, or maybe some rose.
Want to add a more substantial dish to the repast that's fast and easy?
https://www.marthastewart.com/313060/tomato-basil-and-white-bean-salad
or maybe a dish of Tuscan beans with tuna to spread on toasted Italian bread slices.
You can switch to a neighboring country's cuisine and go French with a giant bagnat Nicoise or charcuterie with pate added to grilled and or blanched vegetables with sauce AKA crudities. A radish butter spread onto toasted bread gives a real touch of authenticy and is fast to make as well as being in season. For dessert, again fresh fruit just like the Italians or if you feel mildly ambitious a stone fruit galette. Jacques Pepin offers a recipe that takes minimal ingredients and time in his seminal book( OK, he's one of my cooking idols ), "Fast Food, My Way":
https://www.food.com/recipe/quick-pear-tart-by-jacques-pepin-476484
You are not limited to pears; most any kind of stone fruit or berries will work.
HTH