When you finish your credits for school or to get a degree you get a diploma and the ceremony is a graduation.
Cap and Gown
A cap and gown will help enhance the display because it is stereotypical that you wear a cap and gown in the graduation ceremony.
The whole reason for a graduation is to get your diploma so you can't have a graduation display without a diploma.
A cake and party favors
When you graduate high school you usually have a party since it is such a big achievement.