A tent
Pup tent or lean-to
a real life example of an octagon is a stop sign.
A pennant is a real life example of an isosceles triangle.
you
An Amicelli box! THIS IS NOT TRUE IT IS HEXAGONAL!!
a doghouse
A solid brick is a rectangular prism. (this one example of many)
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.
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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.
Real life is a real life example!