Asking children to describe the type of world they would like to live in as adults serves as a good writing prompt for second graders. Alternatively, a teacher could ask the children to describe dreams they've had.
Bringing in a photograph or painting and letting the children invent and write down a story based on what they can see and imagine provides another engaging option.
Children are often the most engaged when they help create the prompt themselves. A teacher may try facilitating a conversation that allows them to have some control over the prompt. For example, a teacher can have each child write down a made-up character on a slip of paper, draw three slips out of a hat and write a story that connects the three characters chosen.
In addition, K12Reader.com has a list of accessible prompts online for all grade levels.