Stop buying your art supplies and get outside. Potential art supplies are everywhere and you are more likely to find something cool and create an amazing, unusual piece with some random thing you found than with something you bought. For example, you can make a collage with rusty beer caps or use broken bicycle inner tubes to make a mobile. There are even artists who make paintings with blood and sculptures with elephant dung. Art can be anything you want it to be and it can be made out of anything.
Size often plays a major role in unique art projects. Some artists try to make the smallest version of a thing, like a tiny book for example, while others, like Koons, try to make the largest of a thing; in his case, for example, the largest balloon animal sculpture. A life-size sculpture of realistically fleshy people made from silicon would be unusual. Using tiny sticky notes to create a large collage would also be unusual.
Other art projects are more unusual in concept than in supplies or form. This type of art can be different from the expected interpretation of art being a visual experience. The conceptual aspect of art is rarely broached until college but it can make for extremely unusual and unique art such as that done by conceptual artists and performance artists. Examples of the type of work include a shoe box that represents "life" or a woman wrapping herself in saran wrap to protest the sexual role of women in society.
While just about any kind of art, including painting, sculpting and drawing, can be made unusual by the types of tools you use, its size or the concept, some types of art are truly unusual. Examples of these types of art include making sculptures out of old cars and refrigerators, building house walls using old soda bottles, pinhole photography -- when you make your own box --- and many, many more. "Unusual" is simply another way of saying "think outside the box." Instead of doing things the way you always have, change it up by incorporating something different into your art by trying new types of art, concepts, different sizes or new materials.