This book covers the basic topics of Materials, Mixtures & Compounds, Energy/Forces/Motion, Earth & Space, Light/Sound/Electricity, Plants, Animals, and the Human Body in a friendly way that will appeal to elementary age students. Then, when one of the topics strikes your child's interest, he can visit the websites recommended to go along with the topic or try one of the experiments.
We recently grew our own crystals out of alum, based on an experiment in this book.
After letting them grow for a few days, we chose the biggest one, and using it as a 'seed crystal', put it in the jar and watched it grow some more.
It was a fun experiment, and though the Usborne Science Encyclopedia seemed a little expensive for this cheap mama, I'm really glad we have it.