GMod really shines with the wiremod addon.... with that, you can even make computers and game consoles ingame...
I particularly enjoy spacebuild maps because of the complexity of having to build the nescessary life-support systems etc. on space ships, space stations or bases in inhospitable environments...
One of the most complex things I made was a laser 3D scanner that outputted to a holographic emitter, however, with good resolution came loooong scanning times.
I own GMod 10, for those who want to try some of the sandboxing, I still think you can get GMod 9 off of Steam.
However, I haven't played GMod in ages 