For low voltage circuits, so that you could show the principle you are looking for with even just a battery as the power supply, and where a beautifully perfect sine wave is not a strict requirement, a 'Wein Bridge Oscillator' or a 'Low Frequency Sine Wave Generator with Quadrature Output' (one signal is a horrible 'sine wave', the other quite good) could work. See AN-31, Op Amp Circuit Collection, section 2 'Signal Generation' for such circuits. This could be rectified with a precision full-wave rectifier circuit or just a diode bridge. Op amp sine wave generator and rectifier circuit could be a way of showing the uses of op amps and AC-DC rectification.
What about a tiny transformer with very low output current capabilities. I have a dual output 220VAC to 12VAC transformer that can only output ~70mA - it is about 5x5x5cm. That would be good for adding a diode bridge and showing ripple vs capacitance, bad for having to make the thing and add fuses, even if it is a small circuit.