To help get you in the mood, I've compiled a list of picnic best bets from around Jersey City, nothing that needs to be kept cold, warmed up, or requires major preparation:
Rice Balls from Koro Koro - individually wrapped, self-contained, and delicious, these are great for snacking on throughout the afternoon. Their salads would be a great addition to help round out the meal. There are also vegan and gluten-free options.
Pretty much any Italian deli in town - the sandwiches are filling, delicious, good to share and easily transported. My particular favorites would be Second Street, Monmouth Street (first post ever by the way), Andrea Salumeria, or Salumeria Ercolano.The only downside is they tend to close early so you're best off getting there before 3pm.
WHOS Gluten-Free is a great choice to pick up picnic-friendly snacks. They carry everything from their own specialty trail mix to gluten-free, vegan brownies. You can pick up from their store, 942 Summit in the Heights (where you can also visit Do You Tea if you feel like brewing closed pitchers of tea to bring along) or from 9 Bar Cafe and The Big Straw downtown.
Two Aprons in Paulus Hook would be an ideal place to visit before a picnic. They carry great sandwiches, simple but filling salads, and have rotisserie chickens available so you can feed a group easily.
Check out Gypsy Grill for hummus platters, veggie appetizer combos, and delicious and easy to carry salads. Their sandwiches are also good, but best eaten warm and can also get a bit messy.
No matter what you bring to the picnic (or any other picnic or outdoor gathering for that matter), please make sure you clean up entirely after yourself, picking up any trash or recycling, and leaving things clean for everyone else who wishes to enjoy outdoors.