Ironically, we sometimes come up with our most imaginative ideas when we have the least amount of time to think. The pressure of time forces our natural instinct and intuition to take over, so that ideas are more spontaneous and innovative. This may also apply to some of the children in your class who might surprise you with the quality of their thinking when working within tight time constraints.
The Design and Technology Association led a successful project with primary schools in Shropshire called Butterflies in My Tummy. A major aim of the project was to provide activities for KS2 children to maximise their risk taking and innovation when designing.
One of the activities, Extending the Range, is an excellent strategy to have up your sleeve when you want children to quickly come up with ideas for what they could design and make. During this activity, children are put in the position of a professional designer and asked to consider an existing range of products.