The Beach boys used the Theremin which use electromagnetic feilds to control pitch with one hand and amplitude with the other. The synth engine was a bunch of tubes capable of spitting out some simple wave shapes. How ever in the video for Good Vibrations, the dude is playing a Ribbon controller. This would work just fine too

The other do dad that is being talked about here used an accually electrical contact or the "ring" to let the electroncs decide what pitch to play.

Others have used light and all sorts of things.

Synths are not restricted to being keyboards, just keyboards happen to be the interface of preference.

I remember seeing an early Serge(?) modular which used copper touch plates for interfacing.

-tek