Pick one thing you remember about them and start there. Here's what to say to a parent, a new baby and a couple.

A story he always told is often the best thing to keep. Tell it the way he told it.
For a new baby, say hello and tell them who you are. They'll hear it when they're older.
For a wedding, tell one story about the two of them.
You do not need a script. Say it the way you would say it to them.