My 3-year old loves picnics! I think that being outside and eating is more like having a snack than eating what I ask her to for meals. And that's probably true.
But you don't want a picnic to be too much work! You want to enjoy the outdoors and the food and the rolling down of hills or picking leaves or flowers (more often weeds in our yard). So how do you incorporate poetry but still have a fun, casual picnic?
Well, you could do rhyming foods.
peas and cheese and strawberries (maybe not all at once)
rolls in bowls and doughnut holes
Hotdogs and ants on logs (celery with peanut butter and raisins) and chocolate frogs (oh wait, this isn't Hogwarts)
You get the idea.
You could make up similes and metaphors to go along with what you're eating.
That actually can be really fun -- especially with young kids because they have the best imaginations.
You could talk about onomatopoeia -- words that are sounds -- and what each food sounds like when you eat it (could get a little gross though)
You could write shape poems for each item in your picnic or acrostics to describe them.
Could be some fun possibilities.