MeshMagic 3D is a tiny free 3D design tool.

This is very much "CAD lite". The download is under 1MB, it unpacks to a single executable plus an installer, and there are no sample models, local help files, or extras of any kind.

Drawing options are unsurprisingly basic. You can add a few 3D shapes - cube, sphere, torus, icosahedron - or draw lines/ outlines and transform them into 3D meshes.

MeshMagic 3D is more effective as a file viewer, and can import both STL and 3DP models. We found this didn't always work with complex models - a few appeared to open, then nothing was displayed - but most simple models appeared in a basic form (you won't see any renderings).

If you do manage to import something then you can edit it, add or delete shapes, maybe extrude a new surface from an existing surface, then save the results as an STL.

Please note, the MeshMagic 3D setup program will by default install the Google toolbar along with its own program, but you can avoid this if you clear the relevant checkbox during installation.