I am in the first physical computing class at the moment, wondering whether or not I can build a quadcopter. My current thoughts are "basically, no". Quadcopters are expensive, the entire point would be to mount a camera on it and get photos of London. a) I'm not sure that's legal, b) it would require mounting an expensive device (my camera) to another complex and fragile device made by me, potentially ending in the demise of both expensive devices.
Another thing I'd like to build is a synthesizer. That sounds fun, can't think of a good reason that wouldn't work, I probably will do that. I'd need to do something to make it not "just" a boring simple digital hardware synth. Maybe a digital control for some analog synth stuff. Maybe digital for CV and analog for audio signal. Sky's the limit. I'll come back to this with a more in-depth brainstorm later on.
This class reminds me of the various "Technology"/Metalwork/Woodwork classes I did in school. Some of those were pretty good, I made a broken guitar once. I'm still considering building an "augmented" guitar - a guitar/bass with an ardunio-like device inside, reprogrammable to manage the circuit configuration (phase and on/off of pickups, what certain knobs do, etc), as well as to digitally manipulate the sound (define how each pickup combines, modulate that combination with knobs, etc.) as well as control output devices on the guitar like LEDs and displays as well as electromagnets playing signals into the strings, actuators to pluck the strings and (this particular one is very unlikely, but...) motorized tuners to make sure the guitar stays in tune throughout performances despite things like unreliable tremolo mechanisms. But, I mustn't forget that I did once try and build a guitar, and despite getting a B in that class, I do not now have a working guitar that I built. I could potentially start with a guitar kit, sets of unassembled and unfinished components for guitars that you can pick up relatively cheaply on eBay.