I have been having a lot of picnic lunches since I was 12; and this has been our major family activity.
I love the independence of of carrying lunch with me, so I can sit down anywhere to eat at anytime I fancy (usually earlier than other people).
The minimum basic requirements can be reduced to a few elements, numbered in order of importance - elements can be added, in sequence, from lower in the list as time and need allow.
The following is what I personally like - naturally, when other people come along, their preferences also need taking into account.
1. A drink. A bottle of tap water is probably as good as anything when you are out and about.
2. Sandwiches (plenty of). Four slices (min) of normal bread, or (better) a couple of doorstep slices (each about an inch thick). Filling - (i) Peanut butter, (ii) Cheese and chutney.
4. Fruits: washed and in a polythene bag: e.g. grapes, small tomatoes, an apple.
5. Something sweet, preferably with chocolate or chocolate, or buy an ice cream when you return to civilisation.
And, of course, a book - always carried (but not necessarily read).
So the minimum picnic lunch would be water and peanut butter sandwiches - although some could include a bit of jam as well, for variety.