The Exploratorium, in San Francisco, has launched Science Snacks, a great collection of hands-on science lessons for middle school and high school students. Science Snacks features activities that can be conducted with inexpensive and readily available materials. Each Science Snack comes with a materials list and step-by-step directions. Science Snacks are also accompanied by a written explanation of the science at work in the activity. Many Science Snacks, like Penny Battery, include video demonstrations and explanations.
You can search for Science Snacks alphabetically or you can search by subject. The subject search is the best way to search if you are looking for an activity to match a lesson plan or curriculum standard that you already have in mind.