You can go to this website and find some cool ideas for science competitions.
"The Elementary Science Olympiad offers over 80 challenging and motivational events which are balanced among the various science disciplines of life science, earth science, and physical science. These individual and team events also offer a balance among events requiring knowledge of science facts, concepts, processes, skills, and applications. While a number of the events are in the form of a general quiz, the majority provide an element of "hands-on" participation allowing the "active" study of science. The emphasis is on learning, participation, interaction, and having fun."