Sometimes, if I've just walked into the middle of a conversation I'll just stand there and nod, saying things like “I agree!” and “Interesting…”
:)

I wait around in the park for a jogger to come by, when they do I light a cigarette and challenge them to a race. When I win they have the most pitiful look on their faces.

Remember kids: Jogging will not get you shape, you will not lose any weight. It's only an excuse to wear silk shorts.

I talk to myself and respond to my own spoken words, but that just helps me relay information, usually. I tend to whistle when I get the chance, and I always air guitar. I sing to music that I really cant (Amon Amarth, Rammstein, Soulfly, stuff that usually requires a deeper voice).

I once shaved my entire head and eyebrows using only a shaving razor, no gel or anything just water. But that was about a few days after I started hearing voices…

I flex my muscles. Not in a cool body-builder way, either - just like small muscles in my legs in patterns. Usually about six or so and then repeat. For no reason at all, except that I quite like to do it. I used to do it a lot more when I was a kid, but not so much these days.

Oh, and I also stick my tongue out at me husband for no reason. Lots. :P I've never done it with a good reason, I don't think. I just like to display my tongue.

I thought of another one. I often put objects that aren't hats on my head and ask, “do you like my attractive hat?”. I most frequently do this with cans of beer if I have occasion to have one, although almost any object will do, as long as it's not a hat of any kind.