Use a ball point pen to write or draw on the bound testicles. This produces sensations that vary from sensual to painful, depending on the amount of pressure applied. Again, the amount of pressure used determines the reaction you will get. You may even try placing the tracing wheel in the freezer for a few hours before hand. Any instrument may be used in this manner, however, you may want to avoid anything that would cut or puncture the skin.
Clamps There are various devices which can be used to clamp the skin around the testicles.
Nipple clamps, clothes pins, and binder clips are just a few useful things that you can use. Leaving these on for a period of time allows the subject to adjust to the pressure that is being applied to the area. However, when the clamps are removed, the experience will be extremely painful. If you use clothespins, try purchasing the plastic ones that have holes in their ends. These can be applied over the entire sack of the testicles and then laced together with string or thin rope.