stuff
A pencil.
A tent
A gaming die.
A real life example is a Toblerone chocolate bar which is in the shape of a triangular prism.
An Amicelli box! THIS IS NOT TRUE IT IS HEXAGONAL!!
The Pentagon Department of defense office building
Pup tent or lean-to
A building would be a perfect example. If you simplify the details of a building, it becomes a rectangular prism. If you specifically mean a "square" prism, that would be a cube, an example of which would be a Rubik's Cube or a few dice.
roof
Real life is a real life example!
Roof of a house