There are so many physical phenomenon that goes unnoticed that life would be so much different without them. From the smallest (quantum phenomenon) to the biggest (cosmological - big bang), there are so many of them. Even one small change to any of the physical constants (say, speed of light) would change so many things.

However, in the spirit of what Kelly Cox has written, I would give a few examples that would hopefully inspire students to think and imagine:

1. Gravity

Without gravity, it would be impossible to walk, or jump, or go skiing, or sky diving. We see all things falling towards the ground, and this has been taken for granted without much thought, until Isaac Newton came along.
If there was no gravity, we would all be floating around and not be bound to Earth's surface, without any atmosphere or even the Earth in the first place. Gravity holds everything together and makes life possible on Earth!

2. Air pressure
If we had no air pressure, our internal blood pressure would actually cause our human bodies to explode. This is why astronauts have to wear spacesuits in space (other than the exposure to radiation).

3. Light as a wave
Light is a wave, thus it undergoes diffraction (scattering), reflection and refraction. Diffraction is what makes beautiful rainbows; reflection is what allows us to see ourselves in the mirror dailly; refraction is what allows us to look through magnifying glasses, microscopes and telescopes to discover the wonders of the world much smaller or much further from us.

4. Sound
Sound is a wave that is carried through air and other materials. Without sound, we would not have such beautiful music nor would we be able to communicate by talking to each other.

5. Energy conservation
Energy conservation allowed physicists and engineers to work out theoretical models and build machines to extract work out of nature. It is a simple principle that allows us to predict how much energy we can obtain out of a machine. The fact that energy cannot be created or destroyed also means that we can't get free lunch (ie free energy without doing work).

Of course, this list is non-exhaustive and can be extended indefinitely. Hope this helps.
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The examples are :

1. Gravity
2. Air pressure
3. Sound
4.Light as a wave
5. Energy conservation.
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