For this demonstration, you'll need 2 student volunteers and 2 identical spring scales.
Hand each student a spring scale. They'll hold the loop end with one finger. Hook the opposite ends of the spring scales together.
Instruct one of the students to pull his/her spring scale with 10 N of force (or whatever number is appropriate for the spring scales you are using; something in the middle of the scale) and the other student to pull with 0 N of force (or 5 N or any number as long as it's different from the first student).
Then let them try to do it.
When they can't get it to work, I usually step in and try to "help". I have the one student pull the spring scale so it reads 10N. Once that one's set, I tell the other student to then pull his spring scale to the predetermined number. As they're doing this, they'll notice that both spring scales are always at the same number, no matter what they do.
It's that whole "For every action there is an equal and opposite reaction" thing. If a student pulls on one scale with 10N of force, the other scale pulls with 10N of force (equal force), but in the opposite direction.
It's a really simple demonstration, but it really exemplifies the "equal" part of Newton's third law.