Gravity.
If it's things to change, then choose anything. Length has already been taken, and gravity will be constant unless you do something extreme. Number of people in the room, day of the week, yelling at it etc, will all have the same effect on its period ie none. T = 2*pi* (sqrt(L/g)) for small angles ignoring friction. Mass, type of string etc will have no measurable effect.
This post was edited by TIMMY213 on Feb 14 2012 11:29pm