Once upon a time I found and brought home some lengths of plastic tubing (intended for plumbing, I imagine). They were very long - up to 3 metres - with about a 1-inch diameter. When you sang directly into one, you found your voice pulled to one of its natural harmonic pitches (the harmonic scale familiar from valveless hunting horns etc.); being so long, there was quite a full scale of these accessible to my vocal range. It was such a strange experience, actually feeling your vocal cords 'snap-to' specific pitches, as if quantised. So, if you want to experiment with tubes/piping.. go long!
If you'll stretch to going electric...
Wire up a basic contact mic, sing into that. Try attaching it to various membranes for a very dirty mic. Also, always keep an eye out in charity shops for kids' toys with sound distortion functionality. Some of those are pretty sophisticated! I picked up this guy for £1 not so long ago, and have been using it in performance. Includes octave shift up/down and a vocoder-type effect.
Another thing to try is cupping the microphone. Cardioid mics use air filters on the sides to eliminate off-axis sounds through phase cancellation. If you block those filters, you change its pick-up pattern (it turns omni). If you're monitoring the input through a speaker, you'll get feedback - and different degrees of cupping will give you varying amounts of feedback or a bit of a ring on certain vowel sounds. The frequency of the ringing of course corresponds to modes in the room, so try changing your position as you do this, with respect to the monitor, and to different corners of the room.
