I would use “the earth”. There are multiple directions you can go here.

Key points:

I have two sides that are as different as night and day.

I can be as violent as volcanic eruptions (be very loud and boisterous here) and as calm and tranquil as the sea (with a very soothing voice). The tone you use can really set you apart. Also use hand gestures, raising fists when boisterous, slow, smooth gestures when calm. Seriously.

I am the most important thing to all living beings and I am often taken for granted. Without me you would have no food, no water, no shelter. Without me, humanity would be floating aimlessly, but I keep everyone grounded. And one day, one person will understand my true importance to our society and share it with the rest of the world. Then I will have a day named after me.

Use a prop. If you can find a small foam earth and paint half of it black to illustrate night / day. If you don’t use this idea, you might want to consider using a prop anyway. A small one. One you can toss aside in the middle of the speech to free up your hands for gestures.