In the case of the Vows Renewal and the Commitment Ceremony, the certificate is for the couple as a memento of the day.
For the Naming ceremony, I hand out Certificates to everyone involved in the ceremony - baby's older siblings, Godparents, Grandparents, special people in the baby's life like great grandparents, and lastly to the baby.
To make the certificates even more decorative, I use special fonts which I download free from the Internet. Here is a list of some of them. In some cases I will use only the Capital, while using a more plain font for the rest of the word.
Check out these Free Fonts and download them for your own Certificates and Programs.