1. Small history lesson (with audio examples)
1. Explain the basic building blocks of subtractive synthesis (Oscilator(s), filters, amp, and envelopes
2. Illustrate how to create some simple sounds using the above
3. Let people loose to try to create some of their own sounds. Maybe have some ideas (try to make a flute sound), along with 'solutions' if they want them
4. Maybe: talk about other types of synthesis
5. I don't know; got any other ideas?

The biggest hurdle is figuring out what synth I'd want to use for this. What I need are:
-2 Oscilators
-Filter with ADSR envelope
-Amp with ADSR envelope
-Headphone output
-Some kind of built-in way to play pitches (keyboard? stylus(ugh)?)
While I'd like to do this with a hardware (knobby) solution, I think that that is probably going to be too pricey (I'm putting a roughly $100/person as the top end of what this can cost). So, maybe a software solution? People here have Windows, Mac, and Linux machines, so a software solution would have to be cross platform.
tl;dr:
Looking for recommendations for a tool to use to teach synthesis that is $100 or less; can either be hardware or software, but ought to provide hands/mouse-on access to basic functionality and envelope shaping. Any thoughts on a 'synthesis for n00bs' course in general are also appreciated.