I have always been a fan of eating out and a picnic is just even better. A few years ago when I lived in LA, I used to go to a park to read and would always enjoy seeing families and other people having picnics no matter if it was summer or winter... of course, LA winters are quite mild.
Whenever you can plan it, chose a site near a tree so you have support for decorations like lanterns, battery operated string lights, or flowers that can create the perfect mood.
Opt for foods that require refrigeration as you may run the risk of your food getting spoiled. If you bring something that needs to be cold, fill plastic bags with water and freeze them so you can put them with you food to keep it cold.
Go for comfort, bring pillows and lost of blankets that you can lean into, keep you from the cold and prevent for you and your food to get dirty or covered from the sun.
Comfort applies also to you food, look for foods that are easy to eat and if your ingredients have water in them, like tomatoes, leave them out and add them when your are ready to serve that way sandwiches won't get soggy or salads get watery. Now go out and enjoy the great outdoors!
Have a lovely day!