Clothespin tag is described in a several places on the Internet. So search around if you want more ideas on how to use this activity. The concept is simple; there are two (or more*) teams each with clothespins attached to their clothing. The first team to take all of the clothespins off the other team wins.
It is recommended that there are at least two clothespins per student and a minimum of two colors. To color the clothespins simply use a "Sharpy" marker. Be careful of any type of ink that might run, especially on a white gi.
At first glance this activity doesn't appear to have much value until students start doing it. I recommend having the students perform this activity in several levels.
This is the easiest method of play. Attach the clothespins to end of the sleeves of their gi tops. In order to get a clothespin, the student must expose their own. Also this level is usually the fastest.
Attach the clothespins on the outside of the gi at the elbow. This change requires students move faster and also be closer to one another when trying to remove the clothespin.
For the last level have students attach the clothespins under the arm at just below the floating rib level. This allows students to block attempts to grab the clothespin, while exposing their own if they are attempting to grab one themselves.
This exercise works on many skills; awareness, movement, timing, coordination and speed to name a few. Usually students will get their heart rates up as well as have a good time. You can also vary the game by changing the clothespin locations or having them in alternate locations. (ie one on the elbow, one on the rib)
*Creating more than two teams requires a 3rd color of pin, but adds to the complexity of the game.