Salad, maybe a chicken salad sandwich, flavoured water and alcohol - any picnic I've been a part of we've always organised our own supplies so that there isn't someone who's like: "I don't like any of this".
With the alcohol mixed, for me anything else is too heavy to eat - especially as we only used to have picnics when it was very hot outside.
Lots of cute little sandwiches, some fizzy drinks. If the picnic was on a day that wouldn't be too hot then I'd bring some chocolate, too. Besides that maybe some fruit?
A picnic with some cupcakes would be absolutely lovely, if only I had some good way to transport them...