Conceptual design dominates the early stages of most creative design processes. During these stages, the designer makes important decisions about the parameters of a model that are aimed at satisfying a set of design criteria. To do this, the designer produces many sketches of various possible solutions. Meanwhile, computer-aided design (CAD) systems are becoming the dominant visual medium used in design practice. However, these tools evolved as documentation production tools and do not support conceptual thinking. This paper presents a list of guidelines for computer support for conceptual design activities on 3D scenes and presents SESAME (Sketch, Extrude, Sculpt, and Manipulate Easily), a system based on these guidelines. Finally, we present a user study comparing SESAME with a conventional CAD package to demonstrate the effectiveness of SESAME.