Sensors for BEAM-style robots

[CSCI_Will]

Hello, all.  My current project (my first custom-robotics project) is building a basic, differentially-steered robot that reacts differently to different stimuli.  Basically it'll be  a triangular chasis with two motors on the front, and a caster on the back.  It won't use a microprocessor: instead, I'm going to use photoresistors to vary the voltage across the motors. My question is, are there any other sensors I can use?  I'm thinking about looking into thermocouples (not that I really know anything about them), but otherwise, I don't really know what to look for.  Ideal sensors would vary resistance according to humidity, sound, or other environmental factors.  I'd also really like it if there were any sort of advice and/or documentation with regards to how to use them, and, unsurprisingly, I'd like to keep this as cheap as possible.  Any suggestions?

[Razor Concepts]

I would not use photoresistors to resist the motors. Use transistors instead - look up simple BEAM circuits that will show you how.