I found a great book - "703 science experiments for kids" - most of them utilize things you already have around the house. The first half of the book contains experiments that are short and easy to do - the latter half of the book contains experiments more suitable and challenging for older kids.

I like how these are divided up into categories. We can do one or three or five experiments in one day on the same basic topic, with the same tools - depending on how much time we want to spend.

I love this book!