Hook one oscillator to the left channel input of the stereo and the other to the right channel. Adjust the oscillator output and the balance on the stereo so that the two each produce about the same volume.
Set one oscillator to around 500 Hz, and slowly tune the other one to match it. You should hear clear beats when the frequencies are close. You may show before or afterwards that the beat frequency is equal to the difference between the two input frequencies.
This is also a great opportunity to show the range of human hearing. You can use just one of the oscillators. Start at some frequency that everyone can hear (say 500 Hz) and have everyone raise their hands. Raise the frequency slowly, and ask people to lower their hands when they can no longer hear the sound. Try continuing the lecture for a few minutes with the oscillator set just beyond your own upper limit of frequency.