Simple circuits for squarewave shaping etc

[michael page]

Hello all,

Im making a 40106 synth and was thinking about simple effects circuits to put in the audio chain.  Obvs distortion ones won't do much. 
Im probably going to do some sub-harmonic thing with a 4040 divider chip.
And probably feed noise in.
And some simple filters.
What other stuff would make nice audio variations? simple ring modulator perhaps?

Any thoughts appreciated.

Cheers

Mike