One idea which left the children buzzing and begging for more was the "Concept Cartoon Challenge". I used the fantastic book by Stuart Naylor and Brenda Keogh, which depicts scientific ideas in cartoon form, with children's ideas in speech bubbles to use as thinking points.
Each day during science week I put a poster of a different Concept Cartoon around the school, which the children had to track down. They competed to find the poster first. After looking at the cartoon, they had to develop ideas of their own.
The best ideas were shared in assembly each day and scientific prizes were given to the best thinkers.
Science co-ordinator at Kingsway Junior School, Watford, Hertfordshire