I love finding new things to do with dry ice! We know that we can explode things with it, but once the pleasure from that fades, or if you need a temporary break, just grab a balloon, and make a sound lens.
If you've spent even a little time in a physics classroom, or an astronomy classroom, or even an optometrist's office you'll know how lenses work. They take incoming light and bend it. The bent light will converge at a focal point. Someone standing at that point will be able to see a bright, clear, convergence of light.
Lenses manage to bend light because light travels at different speeds through different media. Lighter media will allow light to go quickly, while denser media will make it go slowly. Imagine you are moving on roller skates across a sidewalk when suddenly one of your skates hits grass. That skate will slow down, while your free skate will keep moving fast, turning you towards the grass. When light hits a dense medium at an angle, it does the same. The part that hit the surface first slows down, and the other side keeps moving fast - bending the light. When it leaves the medium - in this case glass - at an angle, one side speeds up, bending it again. The lens makes use of this.
The bending doesn't just happen with light. It also happens with sound. And you can make use of this the next time you get your hands on some dry ice and a balloon. Dry ice is frozen carbon dioxide. Carbon dioxide is denser and heavier than air. Fill a slightly oblong balloon with the gas that comes from dry ice. (Be careful not to put the dry ice in the balloon, or if you do, let gas escape from the balloon until you know that the dry ice has completely sublimed. Leave a chip of ice in the balloon and it will likely explode.) Put some very quiet object, like a ticking watch or a quietly-playing radio on one side of the balloon. Then slowly bring your ear closer and closer to the other side of the balloon. At one point, the sound should suddenly get loud and clear. The balloon focuses the sound, acting as a sound lens.
Okay, it's no giant explosion, but I still think it's pretty cool. (If you want the giant explosion - take the dry ice, put it in an empty soda bottle, seal the bottle, and sink it in the nearest pool. Then maybe listen to it through a sound lens.)
Top Image: Mikk Mihkel Vaabel