I do that a lot, especially on large size projects. No need to be fancy, often just a big piece of cardboard with the proposed piece drawn in with magic marker.
Its very hard for people to visualize sizes and proportions.
People usually think that a space can accomodate almost twice as many people as it really can.
You can save yourself and your client a lot of trouble by just putting some plywood or cardboard on sawhorses and trying out place settings. Expectations become more realistic very quickly.
I always prototype a chair. If you don't know how a person's body will fit a design, you are likely to fail. Key variables are how high the seat needs to be and where the lumbar support should be. Also, be sure the angle on the back support is appropriate for the desired function, e.g., a dining chair has only a slight recline, while a lounge chair may work best with a large reclining angle.
A little bit of early planning can give a customer the most comfortable chair they have ever sat in.