What should I let students do when they finish exams early?
Why were the students leaving early disruptive? Under the rules of every institution that I've been at you are under exam conditions until you leave the room. They should be utterly silent and respectful to other students or face the usual penalties for breaking the rules of exam conditions.

If you're not happy with them leaving early I'm not sure why you would do anything to occupy them. Make them sit in silence and wait for the the exam to finish. Boredom never killed anyone.