I bite my nails and lips when bored or stressed.
When I'm alone and concentrating hard on something i repetitively tap my head off either my desk or a door post - just gently, and with a regular rhythm. My housemates have always gotten used to this, eventually...
If I'm really confused and distressed (usually a major social stress that I don't understand - e.g. the end of a relationship) I will violently hit my head against a solid wall - although it causes pain, it weirdly focuses my mind and helps me to logically approach the problem. Usually in these cases I will force myself to cry (I have a series of songs that I use!) as I find that triggering normal human emotion is a helpful stimulant, and again helps me to think logically.
Up until a few years ago, if I had become a bit "over-stimulated", I would sit on the floor of the shower with the water running, for hours, rocking back and forth in the nice calm water.
I rub my ears when I'm over-stimulated - it makes me very drowsy. My fiancé now does it to make me go to sleep
I hand flap very briefly when excited.
I do a weird finger movement (like air piano) when concentrating and excited at work (when I can't do the head tapping thing).
I will also sit in any dark little corner at home that I can, usually in competition with the cat. I usually do some head banging or rocking there.
Holy crap, I really should have been diagnosed years ago....!