The Annenberg Foundation sponsors this site (you can find more free resources from them at www.learner.org), and there is a section of Science Interactives here. Although most of the activities said grades 9-12, my “research” indicated that much younger children would enjoy/learn from them. For example, there is a section on the science of roller coasters in which you learn the physics behind them and design your own. There are lots of other choices, ranging from DNA to volcanoes.
Science Mysteries: Mysteries are a natural way to combine logical thinking and problem-solving with engaging stories, suspense, excitement and fun. Mysteries written with a focus on science content and methods are an ideal way to immerse students in the learning process http://www.sciencemystery.com/