The easiest way to get students engaged is to have them build something.
A simple vibrating motor "robot" can become a physics lesson and math lesson and art project and writing project (e.g., write a story titled The Land of The Lost Robots).
Abdul Wahab Malik from Pakistan has assembled a number of easy to perform science experiments.
One of our favorites is boiling water in a paper bag. (Our webmaster used to do it using foam coffee cups as a kid.)
Make A Solar Powered Hot Air Balloon
Learn how you can make a solar powered hot air balloon with just tape and black plastic trash bags. An interesting project for a sunny day.
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PicoBoard, brought to you by the imaginationists at MIT, are designed for creating art with colored lights, sound, music, and motion. Perfect for younger students.
You can make Cornstarch Monsters, otherwise known as oobleck, using corn starch and speaker. This post shows you how to do it and includes a video demonstration.
Load a tone generator app onto your phone, plug in the speaker and experiment with different tones and waveforms.
Bill Nye, the Science Guy, has assembled 40+ simple & fun experiments you can do at home or in the classroom. Create everything from a simple stethoscope or barometer to showing how water sculpts rocks or the Doppler Effect.