Do you mean posh picnic for show where you’re not carrying it far, or practical but delicious picnic when on a day out?
Practical picnic - can’t beat pre-made sandwiches, decent crisps, some kind of delicious cake, tub of soft fruit. Done. Buy an ice cream later.
Posh picnic - never ever a pre-made sarnie but plenty of crusty French bread, deli meats, cold roast chicken and cheeses, prepared salads (potato, beetroot, Mediterranean roast veg & cous cous, pasta/rice, green), ripe tomatoes, quiche or tart, dips like hummus (fancy varieties of), and so on. Something homemade that’s faffy but fancy like proper Scotch eggs or Spanakopita or a Tarte Tatin.
Both stand and fall on decent drinks to wash it down with.
Basically my enjoyment of picnics is directly in proportion to practicality/time spent making. If the picnic is the entire point of the day, go big. If it’s an adjunct, ruthlessly practical, quick to prep, won’t be a pain to clean up later (worst part!)