What is the purpose of a prototype?

- helps us to find good/bad ideas.
- helps us to imagine what a game will be like, without properly making it.
- helps us better communicate our ideas to others.
- helps us to brainstorm, as the prototyping process can lead us to new ideas and directions.

With that in mind, define objective: don't prototype just for the sake of it... think what you want to achieve with your prototype?

genre: most games fit into a genre or some combination of genres, and prototyping can be done in that context.

prototyping specific mechanics: can use placeholder art or even boxes. Doesn't have to look pretty.

prototyping physics: eg, procedurally generating mazes, or experimenting with crowd physics. These can be a self-contained thing - as in - they can be explored without being made into a game.

prototyping mood: to establish a unique look and feel can be as important as the specific mechanics. concept art. mockup screenshots can be useful too, and they don't involve any programming.