I would like to measure the capacitance between two random pieces of metal. Also, I would like to monitor the changes in such capacitance as people walk by those metal objects. Can a cheap off-the-shelf meter do this?
Another way to detect such capacitance changes is to build the capacitor into an L-C oscillator (such as from a radio) and measure its frequency. RF oscillators are usually better than 555 timers if the capacitance is much below 100pf.
You may have to compensate for temperature and other causes of frequency drift but these are usually relatively slow compared to the effects you want to measure.
A typical way to measure small changes in capacitance is to use an AC bridge circuit. My experience with those has been less than stellar, and I avoid them.
A better way, IMO, is to use an oscillator with a duty cycle or frequency that is linearly dependent on a capacitance, and measure the timing parameters of the resulting waveform. A 555 timer in astable mode is an OK way to do this.
That would serve as fine glue circuitry to put into a capture-compare peripheral on a microcontroller to suck the timing information out of. A dirt cheap 8-bit 8-pin PIC would be up to the task.