The most obvious are sandwiches. You can make finger sandwiches very quickly, and they can have all sorts of fillings. You could also put salads together pretty easily, make mini savoury tarts or for pudding, some fruit tartlets. A fruit salad is also a good choice if you bring forks. Another option is to make some pasta, and then have it cold at the picnic. If you want to make your own drinks, try smoothies.