This file contains two dining chairs which become invisible once you place them. They use the basegame mid dining chair with the wooden back and blue seat.

One is chair height and another is offset to look better with beds and other objects. Place them on top of another object you would like sims to sit on.

The chairs are somewhat intelegent and will not allow sims to sit if the object they are on top of is in use, and also will flag the object they are on top of in use to try to discourage sims from using it while another sim is sitting on it.

Some minor issues...

1. It does not look great with every possible object, but there's not much to be done about that.
2. A sim can only use one chair on one object at a time, even if you place more than one object on a multitile object. This is because the chair cannot divine why the object is placed on is in use and the other chair will have already marked it in use.
3. Some funky siutations the objects are not correctly marked not in use if a sim fails to sit, however this could be related to a Maxis bug in the game engine. Currently looking into various workarounds.